Large-scale multi-reference configuration interaction calculations are employed in order to calculate the vertical electronic spectrum of C-4 in its linear and rhombic geometrical conformations up to 6.5 eV. The recently measured transitions of linear C-4 at 0.93 eV ((IIu)-I-3 <--X(3)Sigma(g)(-)) and at 3.26 eV ((3)Sigma(u)(-) <-- X(3)Sigma(g)(-)) are confirmed. The first allowed transition of rhombic C-4 is calculated at 2.15 eV (B-1(1u) <-- X(1)A(g)) and this transition should help to identify rhombic C-4 since there are no transitions of linear C-4 in this energy region.