Personally I prefer the ER-6i.
The ER-4P is a higher-fi piece of gear, but its tonal profile always sounds 'off' to me - I get through a song or two and then I start feeling somewhat uncomfortable; I keep wanting to EQ it, or something like that.
The ER-6i may not have the same objective quality as the ER-4P, but it still sounds pretty terrific, and it never sounds 'wrong' to me from a tonal perspective. It is my unamped portable earphone of choice, by a long shot.
In other words I find the ER-4P's specific type of imperfection more difficult to deal with than the ER-6i's type of imperfection, so I like the ER-6i better. But you could be just the opposite.
Now the ER-4S, that I LOVE. Absolutely outstanding. If you think you might one day spring for the P->S converter, and won't mind having a plain old 4P in the meantime, then by all means spend the extra for the 4P.