Upgrade Shure E4 to newer or get ACS custom buds made?

The E4c/SCL4 is still a good IEM, but I believe it has been surpassed in some ways by newer BA and dynamic IEMs, especially in those which can provide a bit more extension in the bass and treble (Shures are still known for their midrange).  The cable tends to be the weak link, so if it is starting to split anywhere, it probably isn't worth getting a custom tip for them.

I think there are a few options.  One is to save for a higher-end IEM (SE530 or 535 with a replaceable cable if you like the Shure sound), or many many others, including possibly the SM3 or something from Sennheiser, which are less expensive since you are in Europe.  Another option around your budget would be the Etymotic ER-4P or -4S, offering similar bass but much more detailed treble and very nice midrange, or Shures SE310, a type of replacement for the E4.  Shure still makes the E4 (now named the SCL4), so the last option would be to continue with what you enjoy and buy another set.

I hope this helps.

The 530s would be a better upgrade since you like Shure's sound signature.

The Triple.fis are a very different, but equally high-end, IEM.  They have a bit of a mid-bass hump, but seem to go much deeper than the 530s, and have a very unique clear high-end the Shures don't possess.  Think of a V-shaped sound, but still very good, where the Shure is good in the bass, excellent in the mid-range, but a bit rolled off (especially compared to the TFPs).  However, neither IEM can be worn down comfortably, both seem to require over-the-ear.

I really like the Monster Turbine Pro (Gold or Copper) as a very very good universal IEM using a dynamic driver.  While I haven't heard the Sennheiser IE7, I have the IE8, which is also very good, but has a mid-bass hump/warmth that can overpower some music.  The IE8 treble is good, but the Monster Turbine Pros seem to extend a bit more and with more clarity.  However, I also enjoy the coherency that the IE8 brings to my music compared to my 530s.

If the Turbine Pros are out of your price range, I still enjoy the original Turbines. The cable does not seem as sturdy as the Pros, but I have had no issues after 18 months of use.  The sound is a bit warmer than the Pros, but the dynamics and clarity are shared with their younger, bigger brothers.

There are many good options today.  Let me know if you have any other questions.

I moved from the E4C to E500 and still love them.  If you love the Shure sound that would be a painless upgrade IMO.

Yes, the Turbine Pros seem to work best worn straight down.