If you're a 4P user, then presumably you'll be missing the treble if you switch to the super.fi or e5c. I'd recommend the Westone UM2. Not only does it have a ton of bass (and enough power handling to give you a ton more if you EQ it), it also has much of the same treble clarity and detail that the ER-4P is so famous for. Unfortunately, it also has some of the same sibilance and harshness, but less of it, so if it wasn't a problem with the ER-4P, then it won't be a problem with the UM2.
Also, there is the iPod bass roll-off issue to consider. If you're using a low-impedance phone on the iPod straight out of the headphone jack (the 4P is very low impedance), then you will - supposedly - have less bass than when you're using the line-out and a standalone amp. Now, I never had an iPod so I never heard this myself, but it is something to take into consideration. Maybe a DAP upgrade will fix your needs. In any case, the UM2 is a nice canalphone to check out and a definite upgrade from the ER-4P in most things.
P.S. Nice photos! Love the cityscapes.