It's all personal preference, but here's my 2 cents.
I went from 5 back to 4 a couple of years ago with no regrets; on the contrary. I have no specific exercises to recommend, only some thoughts:
- Limitations and constraints stimulate creativity. I think Jaco said something similar. Personally I found the added string a bit of a distraction, and I used it less and less.
- It's a myth that there are some tunes that can't be played without a low B. The 4-string still has ~3 octaves-worth of the same 12 tones. I play tunes in Eb, no problem.
- When it comes to supporting my groups, rhythm and groove trump the added 2.5 tone range, so if you groove, you can do so on a 1-string bass...
- The lower frequencies need to be amplified properly, regardless of the quality of your bass, it's scale, or the pickups. If you don't have a good amp set up, your lower frequencies will likely lack clarity.
All that said, if you really miss the low B and enjoy playing that much more on a 5+ string bass, then don't give it up. Do whatever makes you happy, which is what will make you practice more and therefore sound better and better.