The only thing I could say is, it's going to come down to an individual choice for you. The good news is, it's generally a reversible process, in that you could put the original shaft back in the club.

By changing the shaft length in any given club, you face the fact of changing the swingweight and possibly your ease of using that club effectively compared to your others. Your 4 iron with a longer shaft might not feel like a sequential part of the set.

Keep in mind, a 1" increase in the length of any club amounts to 6 points of swingweight. That could slow down your swing with that club enough to throw off your distance gaps.

Good luck with your experiments...