I guess the key word is "the amp is stable".
Most amps should be stable into 2 ohms.
As long as the amp can handle 2 ohms load, the amp could output more power into 2 ohms. But if the amp can't handle 2 ohms load, then it would be in trouble.
Thinking out loud here:
Two 4-ohm speakers in parallel = 4 ohm/2 = 2 ohm
Three 4-ohm speakers in parallel = 4 ohm/3 = 1.33 ohm
Four 4-ohm speakers in parallel = 4 ohm/4 = 1 ohm
Two 4-ohm in series = 4 + 4 = 8 ohm
Three 4-ohm in series = 4 + 4 + 4 = 12 ohm
Four 4-ohm in series = 4 + 4 + 4 + 4 = 16 ohm
My POV on this - wiring in parallel may get you more power output if your amp is stable, but wiring in series is probably safer and easier for your amp to handle, assuming you don't listen to crazy volume that would require too much power.
Yeah, I am kind of a chicken.