As an "outside observer" (I'm only TH8), all I can evaluate are the various points of view I see expressed in the forum. The overwhelming consensus I seem to be getting is that air attacks have an edge on ground attacks, at least currently. Lavaloonion was already considered stronger than any ground attack, before they made level 13 cannons available.
So it seems to me like a buff to ground attacks is due. Yes, the game shifts back and forth, so air attacks will rule for a season, then ground attacks.
Anyway, it seems to me like giving level 4 pekkas to TH9, from a game balance perspective, is a good move. It wouldn't be OP. It would add variety.
But, badfish brings up a good point. It seems like there is a reasonable balance in the amount of research (number of upgrades, cumulative time) at both TH levels. According to the wiki:
http://clashofclans.wikia.com/wiki/Cumulative_Costs
TH9 has a total of 155 days of upgrades, versus 215 days for TH10. However, if you only look at troops, then it's 143 days at TH9, versus 158 days for TH10.
If you give the level 4 upgrade to TH9, then the troop research time at TH9 goes up to 157 days, versus only 144 for TH10.
TH8 is already
very heavy on research time, so moving upgrades down to TH8 to compensate is a non-starter. Can't be done.
Which means, the only way you can move pekka 4 down to TH9 is if you add more troop research at TH10. In other words, once they release a new troop at TH10, with upgrades, or they add new levels to existing troops, then you can talk about moving pekka 4 down to TH9.
Or that's my opinion anyway.