On a side note, B5M idlers do fit the YZ-2 4gear and thus the YZ-4 indeed. I compared idlers when I built my YZ-2 coming from the AE car and kept the spares when I sold the B5M and later the YZ-2. So far the NIP AE idlers are happy to sit in their bag next to the single Yokomo idler. Since destroying an AE idler in a Redworkshop gearbox on a killer track in a stock race that required you to go full throttle after an awkward jump to be fast more than a year ago I have not experienced any gearbox issues. If you go through idlers like biscuits as has been said, there is always the alumium idler gear option.
However, if you are a big Kyosho fan and dislike BD cars in general, I'd suggest waiting for the ZX-6.6. Although it comes with ball diffs and probably again the plastic steering and rear suspension mount parts, it seems to be a really great kit and I'd buy one myself if I would not have decided to limit my RC endeavours. I think I'll content myself with a new stadium truck for old dude truck racing instead and from time to time run my YZ-4 at meetings with long waiting times between the runs.