Deore, Zee, SLX, XT, Saint, XTR levers from newest generations and 2 previous gens are cross-compatible (with calipers) within the same group and cross-group. They share same diameter master cylinder and when pressing the trigger, the master piston travels the same distance.
If you see/read reports about bigger master pistons in the levers for 4 pistons calipers or that more oil is needed to push 4 pistons is not true at all.
The only thing that's true is that inside of larger calipers themself (with 4 pistons), there is more oil, which is to be expected.
XTR BR-M9120, Saint BR-M820, XT BR-M8120/8020, SLX BR-M7120 and Zee BR-M640 have same size pistons (2x16 mm + 2x18 mm).
P.S. - Magura MT5, MT7 and MT Trail share same caliper body and have 4x17 mm pistons.
XTR BR-M9120, XT BR-M8120 and SLX BR-M7120 are redesigned for more straight oil root inside the caliper, which results in better response to the lever trigger actuation (no matter which lever gen and group).
+ It's also much easier to bleed (air-free) vs. Saint/XT ("old")/Zee, where the bleed nipple is parallel to the banjo and thus it's more complicated to bleed (air-free), because in the further part of the caliper some air can still be trapped.
+ One can adjust the direction of the banjo, which is really helpful vs. Saint/XT ("old")/Zee, where the banjo is in a fixed position and like 2 times longer. So depending on the frame caliper mounting, it can face pretty down and thus require the hose to be bent up pretty hard right after the banjo end.
For modulation difference, they slightly change/rework the curve of the Servo-Wave of the levers of different gens/groups.
The choice should be pretty obvious!
What I don't know is if BR-M7120 have ceramic pistons (like XT), but judging by Zee having them, I would say yes, because Zee is SLX level.
Non-finned pads are all cross-compatible.
Finned Saint/XT ("old")/Zee are not compatibe with XTR/XT (new)/SLX calipers.
Resin pads for 4 pistons calipers contain more metal than those for 2 pistons calipers.
+ They transmit heat more to the disc/hub vs. metallic/sintered pads, that transmit the heat more to ceramic pistons and then to the oil, resulting in wandering bite point when braking for prolonged periods + very noisy in wet.
Best 4 pistons resin pads for Shimano are Trickstuff 260 Power+.
They have better initial bite power and better maintain it, when braking for prolonged periods + wear out less than Shimano resin.
Right now I'm on Saint calipers + XTR trail (9120) levers with Trickstuff pads.
I'm planning to move to new XT or SLX calipers.