Which IEMs would you suggest?

well bar fit what do you like about the S4 and what dont you like?  what direction do you want to go in?

that aside of the ones you mention the custom 3 sound best but have a god awful cable

well not heard the er6 or the se315. howoever i dont believe the 315 are worth their high price, new = price premium.  plus assuming they are little changed form the predecessors id say the PL-50 are a far bettor deal.

the ue700 = ear stabby stabby.  im not a fan.

sf5 is not bad given its new low price litle treble happy but pretty well controled.

actually id take the PL-50 over them all bar the custom 3 which is great sounding.  maybe also look into the Q-Jays too, they equal the kc3 but offer a dryer presentation and have a decent cable.  depens on budget and price where you are really but the pl-50 gives huge bang for the money

To  my ear, the Shure SE315 sounds just fine.  They are definitely worth $200.  The sound is neutral and pleasant.  Now, I'm not saying it's the best out there, but for $200, they are just fine.  I own the Klipsch S4 and am happy with the bass that they produce, but everything else sounds just a bit muddy to my ears.  Not too fond of the treble it has, since it sounds a little muffed.  Good for Hip-Hop style of music, but not for classical or jazz.  This is my honest opinion.

I agree, I have SE315s and they are definately worth the price. they have good detail.