I'm not so sure - especially after the huge backlash for the iPhone 4S when all they did was up the spec a little bit (which made no difference for 99.9% of users) and include a software update for Siri so that they could claim it had new features (it didn't). I know that they still had record sales, but there were many hundreds of thousands of previous iPhone owners like me, who's reaction was more "wtf, that's just lazy apple..im saving my £500 for a real iPhone 5".
The 4 has a great design, and it does feel the highest quality out of any phone on the market due to the glass finish and total lack of cheap plastics, but it can be improved and changed in many ways and still keep that iPhone image. Liquid metal rumours make sense as there is a strong rumour of them including a [partly] metal back again (like with the first iPhone) which would be a great move imo.
We shall see what they do..but I for one am hoping for an exterior redesign this time around rather than a lazy spec bump.