Plato was an interesting man, to say the least. He is widely known for being a student of Socrates who went on to become one of the quintessential philosophers of Western canon, and even went beyond being a thinking man, instructed in grammar, music, and gymnastics as well. He was also a wrestler said to have competed in the Isthmian Games and was dubbed by his coach "Platon" for being broad. Whether this meant fat or strong, I cannot say, but the Greeks were very focused on fitness so I believe he was the latter. 

Plato, like many other philosophers of the time who looked to improve themselves and their fellow man, seeked eudaimonia, which is well-being achieved through excellence on account of good moral thought and conduct, and the tools for achieving these goals were called the virtues. In Republic, Plato established the four cardinal virtues as such:

The first of the virtues is prudence, the capacity to govern yourself using reason. Prudentia in Latin implies foresight, and although nobody can actually predict the future, the wise can tell what is virtuous and what is vicious in the given context of an action or situation. In modern times, prudence is also a synonym for cautious, but this is not the exact meaning of the virtue. 

In a sense, the capacity to tell what is good from what is bad is the most important of all the virtues, for it is the way you can find the others, and you need it to prevent your vices from overpowering you. 

What exactly constitutes prudence? Knowledge is pretty important, but you can know just about everything and be unwise. Experience is also important, for situations you have lived in the past can be a good point of reference in similar predicaments. Common sense is vital to wisdom, for many times complex problems are created from simple ones and can oftentimes have simple solutions themselves. 

There's no real guideline on applying wisdom, but before you make any decision you must stop and think. A few questions that can help you are:

Historically, there are many people who are considered extremely wise. Socrates was one of those, for in his denial of being a knowledgeable man, he prevented any humiliation if his knowledge was proven wrong. Siddharta Gautama, the first Buddha, was also considered wise for he let reason dictate his decisions, not his ego. Confucious said that wisdom could be learned in three ways: By reflection, which is noblest; by imitation, which is easiest; and by experience, which is the bitterest. If you look at the lives of these men, you will see that applying wisdom is all about stopping to think before you act. 

The only way you can prevent a vice is with a strong willpower that will hold you back. As they say, moderation is the key. This can go from substances like drugs and alcohol to not allowing your emotions to take control of you, manifested in non-violence and forgiveness. 

In most esoteric philosophies you will see the notion that letting your desire for anything take control over you will take you away from the path to virtue, nirvana, heaven, or any other representation of the ideal world. 

There are multiple areas in which you can exercise temperance, and if you succumb to your hunger without moderating yourself in any of these, you will see dire consequences. 

The trick with temperance is to look at your own life objectively. If you're wasting your days on a couch watching T.V. or surfing the web, if your scale is going up and your clothes get tighter, if your mind seems to be on a downward spiral and you feel your intelligence slipping away or your happiness fading, if you're running out of money because you always eat out or you're consuming (sometimes illicit) addictive substances, then take a look at your habits and change them, either by yourself or with professional help, like therapists or counselors if you have access to them. If you're paying attention the relationship between temperance and wisdom becomes clear: You need to understand the consequences. 

Inner fortitude is very important for you to have courage. How willing are you to go through pain, danger, uncertainty, intimidation, or hardship? If you are able to go through these willingly in order to do the right thing, then you are corageous. 

It is probably the most difficult of the virtues to cultivate because human nature dictates we stay away from pain and hardship in lieu of pleasure and easier times. Perseverance is, in a way, the manifestation of courage through our actions. The lack of it is, of course, cowardice, but one must not be excessive either or one risks being reckless. This is where courage is tied in with temperance and wisdom. 

Courage is not something that will come immediately. You must build yourself up as a person in every aspect of your life, and courage will come along with self-confidence. The biggest impediment of this virtue in modern life is the comfort zone; when you get so comfortable with where you are in life you are unwilling to make a change because you're so used to the routine you're in that anything different is unappealing to you. If you want a different life, you must live it differently. 

I'm not telling you to drop out to work from your garage, or to step up from that couch and go run a marathon. Remember your temperance. Plan your projects, but don't overplan lest you be stuck there, and take it one day at a time. Run a couple of kilometers at first and add another one every week. Excellence is a habit. 

I won't say this is the most complex virtue of them all, because every person handles different aspects in their own way, but Justice is definitely one that is hard to agree on with others. Let's take it one step at a time. What is fair? What is righteous?

What makes justice difficult is that it regulates how you handle yourself with others. It is the moderate point between selfishness and selflessness, and it requires looking at situations objectively while still being charitable to a point. We can start by looking at the concept of equity; not necessarily equality, but a situation in which different parties are entitled to a proportional claim and are thus equally satisfied. 

Real life is never that simple, though. There are two extremist sides to this coin as well; the lack of justice being cruelty, and the excess of justice being a bleeding heart, or what we call in this day and age the Social Justice Warriors, people so concerned with helping out those who they consider opressed that they themselves end up opressing others. 

There is a belief called moral relativism, which some people use to say that there is no culture better than the other, but I do not believe this is the way to go with justice. I believe that there is an objective good and an objective bad, but we cannot know for sure what constitutes them until we look at the context. I will never support any culture that intentionally treats people unfairly, though, and always will do my best to help educate and change humanity's ways for the better. 

Justice requires you to look at every situation objectively, just as temperance and wisdom do. Courage is also a necessity because justice requires you to have a strong system of ethics which you believe is right, and you need to stand up for it, but what is fair for you might not necessarily be fair for me. There are a few points where you can begin, though. Avoiding hypocrisy is a must; you cannot say one thing and then contradict yourself. Nobility, as in protecting those who are weaker than you are, is also good, for not everybody has the means to do so. Stand up for what you think is right and always question whether what you think is right or wrong is actually so, and you will generally be a just person. 

Normally, you won't have to use one of these virtues individually. Eudaimonia is about constantly using all of these together to the point where your life becomes better and better as a consequence of you being a better person in every sense of the word. Like I said, live the life you want. If you want to be intelligent, then spend your time absorbing knowledge from different sources. If you want to be pysically strong, spend your time at the gym or doing some other form of physical recreation. If you want to help those that can't help themselves, volunteer. Always remember to think about the consequences, make sure you don't indulge in excess and make sure you don't slack off, always stand up for your beliefs, live by your own code as long as your code is good, and be fair to others. 

Read more about Eudaimonia here:

http://positivepsychology.org.uk/the-concept-of-eudaimonic-well-being/