Question

why cant a carbon form a covalent compound with another carbon as C4

Answer 1

carbon do forms four bonds in case of diamond which is one of the allotrope of it...but it has only three bonds in case of graphite and the excess one is responsible for conduction

Answer 2

See the Mendeleev's Periodic Table. C lies in 4 series and had 4 electrons in outer orbit. To be a saturated element the outer orbit must have 2 or 8 electrons so in CH4 , one C atom create co-valent bond with other Hydrogen with 1 electron in outer orbit.