Why can carbon never form more than a total of four bonds?
Carbon can't form because if a carbon atom forms four bonds, it has a full valence shell and can't form any more bonds.
There is a rule in Chemistry called the Octet Rule: An atom is most stable when it has a full valence shell (8 electrons in its valence shell.) Carbon has 6 electrons, two in its inner shell and four in its valence shell. When carbon takes four electrons from other atoms, in which it forms ionic bonds, it has a full valence shell, so it is unable to from any more bonds.