I got following Bit pattern:

1000 0001 (129)

I now want to set the last four bits after my favor (1 - 10, 0x1 - 0xA):

1000 0010

or

1000 1000

I have actually no idea how I can accomplish this. I could read out the first four bits:

var buff = new Buffer(1);

buff[0] = 129;

var isFirstBitSet = (buff[0] & 128) == 128;
var isSecondBitSet = (buff[0] & 64) == 40;
var isThirdBitSet = (buff[0] & 32) === 32;
var isFourthBitSet = (buff[0] & 16) === 16;

var buff[0] = 0xA;

if (isFirstBitSet) {
    buff[0] = buff[0] & 128;
}

and map then on a new one but I think it is self explained that this is crap.

You can set the low four bits of an integer by first ANDing the integer with 0xfffffff0 and then ORing it with your four-bit value.

function setLowFour(n, lowFour) {
  return (n & 0xfffffff0) | (lowFour & 0xf);
}

Note that JavaScript doesn't really have an integer type. The bitwise operations force the values to be integers, but they're really still stored as floating point numbers.

edit — I think it actually works too :-) setLowFour(1025, 12) returns 1036. How's that for unit testing?