The restriction is not a concern when comparing compounds I and II since both conjugate bases are aromatic according to the non-inclusive approach (both are cyclic, have a conjugated pi system since all atoms in the compounds are sp 2 -hybridized according to the non-inclusive approach, and have a Huckel number of pi electrons). It becomes prevalent when comparing compounds III and IV, though, specifically in regards to the aromaticity of the conjugate base of compound III. Upon deprotonation of compound III, the conjugate base's carbanion is sp 2 -hybridized based on the non-inclusive approach (3 sigma bonds + 0 localized lone pairs = 3, sp 2 ) which makes the compound antiaromatic. If we were able to consider the carbanion as sp -hybridized under the inclusive approach (3 sigma bonds + 1 lone pair = 4, sp ) then the compound could adopt a more stable nonaromatic configuration, but since the non-inclusive approach is the only one we can utilize when considering aromaticity following deprotonation the compound is antiaromatic. This makes compound IV more acidic since its conjugate base is nonaromatic (which is more stable than the antiaromatic configuration of compound III).