We know that in butadiene there are 4 MOs. This is the MO with one node:
My question is: Why, with one node, can't we have something like this ?
A good way to think of this is by looking at forming butadiene from the MOs of ethene, which are an in-phase and an out of phase combination of p orbitals. If we looked at all the combinations of these two MOs of ethene, we obtain the MOs for butadiene