Will the new Apple operating system (i os 5) work on the Iphone 4?

I'm looking to upgrade phones. Would it be smarter to wait for iphone 5 that will almost positively run ios5 or should i get the iphone 4 now? I like the ios5 better than the current operating system. Will there be a large difference in the actual phone or just the operating system? Sorry if this seems like alot but i need help!