The present scaling system works as such, across the difficulties, the pets get a substantial boost in HP. But seriously speaking, playing as a very high level pet user, while in a lower difficulty in Act 4 can be frustrating. Especially if you are a pet centric type of user. That being said, how about scaling the pets differently ?

Note that I am not talking about giving the pets an additional boost, but rather talking about how pets should scale.

Hence, let take this dummy summon.

Say this:
Pet X, at Maximum skill level of 20 at Level 20.
HP 20 000
MP 10 000
Int 250
Str 100
Agi 250
Damage 250 to 350.
The attribute scaling, say HP could be like this:
PL == Player LeveLs. Min is 1, Max is 65.

SL == Skill Level. Min is 1, Max is 20.

DC == Difference Coefficient. It is a value between 0 and 1.
DC determines whether skill level or player level should be making the larger difference in this particular attribute. In this case, I propose that player levels should matter more.

Be == Baseline HP.

[PL x DC + SL x ( 1 - DC ) ] x Be = 20 000.
If u solve this equation with PL = 65, SL = 20, and assuming DC is 0.9, solving gets Be to an approximation of 330.579

Now assuming that Player's level at the end of Act 4 Normal is about 35. And he manages to hit a skill cap of level 18,

The pets HP would be about [35 x 0.9 + 18 x 0.1] x 330.579 = 11K HP approx.

Say now, that Player's Level at then end of Act 4 Epic is about 50. And he manages to hit a skill cap of level 20,

The pets HP would be about [50 X 0.9 + 18 X 0.1] + 330.479 = 15471HP.

The main point is this, the player's and his skill levels should be the only factor that determines the strength of his pet. This ensures that its scales more evenly, rather then a dramatic boost between difficulties.

The formulation can once again easily change, based on what are the ideal values should be. Once again, these are all just conceptual ideas.