I think 5 IPCs would be a good price for a SPG unit.
It should NOT provide a boost to tanks. I agree that tanks attacking at 4 would be too strong. Also, thanks to HBG, we are getting some heavy tanks available, at least for Germany (Tiger) and Russia (KV-2). Those would be your attack @ 4 units. I think heavy tanks should be Attack 4, Defend 4, Move 1, Cost 8, NO Blitz. The movement of 1 and no blitz are because they were so big and heavy, sort of a trade off for the heavy attack and defend power. Also thinking of the idea of making them 2 hit units to make up for the extra cost. They get repaired if they survive the battle, like BBs in 1942.
As for SPGs, they should provide a boost to infantry and mech infantry. Finally we would have a unit that could keep up with the mech infantry so mechs wouldn’t loose their boost provider when they speed off away from regular artillery. Now if you want a pair for attack and don’t need the speed, you spend $7 and get an artillery and an infantry. If you do need the speed, you spend $9 and get a mech infantry and an SPG. Great pairs.
One Question: Would an SPG be able to blitz like a tank?