Question: I need to remove every fourth byte in the `byte[]` I have. 
Is there some built-in function that could help me to do that or do I have to loop over the array picking and moving each byte one by one?

Similarly what what if I need to place 0 after every three bytes from `byte[]` is there a way to do this faster than manually?

Answer: 
Coding it by hand is likely to be very fast, and the bottleneck is the cpu-to-memory bandwidth.

One core can easily max out the memory bandwidth on a modern machine, so multithreading is unlikely to help since the work done is very straightforward (a single read or write per 4th element.)
The fastest way would probably be to create a wrapper to translate indexes instead of creating a new array.