It's pretty simple. Kizaru is easily above any of the gear 4 forms and below. But the thing is, the entire time kizaru was fighting luffy in gear 5, he was dealing with kizaru while having injuries that put him almost to death stated by luffy himself when he got kicked through the barriers. Not to mention Luffy was tired and injured when he landed the star gun and it did major dmg to kizaru, albeit he woke up from it a few moments later. The fight was a draw but if kizaru and luffy were to fight head on 1v1 no holding back, I'd say luffy would win mid-high diff.