Question

Why carbon can't form 4 bonds with another carbon?

Answer 1

The accurate answer to this question requires a concept taught in class XI. Since it can not be covered in this answer, for it's quite big; satisfy yourself with a simpler answer for now:

For making four bonds, eight electrons would have to be squeezed in and such a system is highly unstable energetically. So, C-C quadruple bonds do not exist.