Ah, the never ending rod length question!
I have lots of #4 rods and what it comes down to is..... YOU. I have a 10 footer for boats, a 9 footer for boat dries/bank Broonies/small Rainbow fisheries, a 71/2 footer in Bamboo for rivers and a 7 footer glass Epic #3 for what started out as just small burns and streams but now seems capable of a whole lot more.
The Epic doesn't have the reach of a 10 footer obviously but is light enough to hold out-stretched and most would deem it too short for most medium / large rivers but it can chuck a line a long way and I love the way it handles the wind.
So, back to the original question. 8'6" would be better off being a 9'. 10' can be a bit specialised but does give more reach for nymphing and dibbling for loch work and will allow for a team of three in a loch. Paul Procter loves a 10 footer
If you can try out a few (beg, borrow, blag) see what suits. Try glass, try carbon, try in a good wind (something we are not short of this year!), try with heavy flies and try with light. It is a very personal thing.
Me, I love my Guideline Fario 9' #4 which the Glen Tanar Rainbows bend double
But the Epic 7' #3 is starting to take over...
Good luck and sorry if you are now more confused!
Euan