I don't know what "sort of hip" means and suspect that IHG doesn't either.
When applied to London "central" can also mean different things to different people. The IC Park Lane is a very pleasant hotel and is well located for many of the things visitors to London are likely to want to do. For a four night stay I'd purchase Ambassador if you haven't already.
If you can be more specific about your wishes I'm sure there will be more helpful replies.
Thanks for the reply! By "hip" i mean easy access to nightlife and other attractions. It's been quite awhile since I last stayed in London, but I remember it doesn't really matter much where you stay if there is easy tube access. From my perspective, Indigo tends to be a little more "hipper" as oppose to the more traditional CP's. Honestly, I could care less about which brand as long as the place is relatively decent with nearby amenities. I'd even go for a decent HI or HIE in the right area.