Number of pistons has nothing to do with pressure, "stopping power", etc. It all has to do with surface area. Not just piston surface area, but also master cylinder piston surface area (bore size). You can get single-piston calipers that apply more force to the rotors than 4-piston calipers, and vice-versa.
Good 1- or 2-piston calipers with the pistons on one side only do not apply pressure unevenly or apply more pressure to one side. For every action there is an equal and opposite reaction, and the slider mechanism efficiently allows this reaction force to be applied to the other side of the rotor.
There is one major factor that hasn't been brought up yet, and that is temperature. I'll make the disclaimer first that this really isn't a concern when choosing brakes for the street, only for the track (road course) where high temperatures are the norm. Temperature is the enemy of brake performance (in general), especially when the brake fluid gets overheated, which gives you a spongy pedal and brake fade. Think of how the heat gets to the fluid -- it is generated at the pad/rotor interface, travels through the pads, through the backing plates, through the pistons, and into the fluid. Along the way, the caliper body absorbs some of this heat and dissipates it.
Comparing two calipers, a single-piston and a four-piston, with equal total piston area, the four-piston caliper can typically dissipate heat better due to the increased piston mass and caliper housing size. The caliper is a heat sink, so bigger is better in simple terms.
Compare Willwood Superlite 4-piston calipers to Wilwood Superlite 6-piston calipers. The total piston area is about the same, giving the same braking force, all other things being equal. However, the 6-piston calipers are what you would prefer for road racing, as they can dissipate heat much better.
The things that have been said about distributing the force along the pad better are true as well, but modern calipers compensate for this well.
The stock early-model Kelsey-Hayes 4-piston caliper is actually a pretty good unit, and is comparable to the Wilwood 4-piston. The Wilwood will deal with temperature better though.