I would say the E4 are worth the $40 difference. Heck I think they're worth $100 difference since that's what I paid when I was in Tokyo and found my E5 had lost the right channel somewhere over the Pacific. But if you can swing the extra $150 or so and you want solid, heavy bass in your IEMs, get the E5. Unless you want to spend $500+ for the professional IEMs, nothing beats the bass of the E5 in my opinion.
Edit: Though I have read that the UM2 has the same bass as the E5 with a more balanced sound, I have not had the opportunity to try them yet. Chalk it up to the lateness of the original post to my forgetting about the UM2.