A carbon atom will always form 4 bonds if it is neutral.
A carbanion (carbon anion, negatively charged) has an extra electron (for a total of 5 valence electrons, like nitrogen). So, it can only form 3 bonds and it has an extra pair of electrons in a lone pair (like nitrogen).
A carbocation (carbon cation, emphatically charged) is feeling the loss of an electron so it has just three valence electrons (like boron), a carbocation has a vacant valence orbital.
Both the carbocation and the carbanion are somewhat flimsy developments for carbon and if conceivable, they will respond to reduce the insecurity and wind up with the nonpartisan valence of 4 once more.
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