First of all when you take the square of 4 you get 16. I presume you meant square root. I would imagine that the algorithm used to find the square root results in 2 plus a very very small fraction which is not shown because it is so much smaller than 2. When you subtract 2 from that you are just left with the fractional part which is, at least on my computer, around -8 times 10 to the -39th power which is damn close to zero.
One of the first demonstrations I had on how computers do math was back in 1971 when it was shown that 1/3 + 1/3 + 1/3 was actually slightly less than one according to the computer.