Cyclobutadiene is cyclic and contains conjugated double bonds. Each C atom is sp2 hybridized. Yet, there are 4 pi-electrons and, hence, cyclobutadiene is antiaromatic according to the (4N) Hueckel rule with N=1. Does ths mean that cyclobutadiene cannot exist? No, it just means that cyclobutadiene does not benefit from conjugation and instead will contain more or less localized double bonds. The structure of cyclobutadiene clearly shows that the molecule is not square but contains two short and two long bonds.