When a single bond is formed between carbon atoms (or any atoms), energy is released. Changing the molecule's electron arrangement to make a double bond releases more energy, but not as much as when the first bond was made. Making a triple bond again releases energy, but again to a lesser degree. However, when a fourth bond is attempted, the overcrowded electrons between the atoms resist the change so strongly that it requires little, if any, energy. This arrangement is therefore unstable, so a quadruple bond will be essentially impossible.
