Most speakers are about as efficient as a toaster at converting electrical energy into sound energy. Their 'power' rating is basically telling you how hot they get (how much power they can dissipate), not how much sound power they output (how loud they are).
A coil speaker (which is what your 4 ohm ones are) is literally a moving coil of wire held between the poles of a magnet. That coil of wire has a DC resistance - 4 ohms in your case. If you stick an AC signal into it, then the (RMS) power dissipated in that 4 ohm resistance is given by:
Power(rms) = (V(rms) ^ 2) / R
V(rms) of a sine wave is the peak to peak voltage divided by 2root2 (2.828)
So to get 30 watts RMS into a 4 ohm speaker you need to drive it with about a 31V peak to peak signal (typically +15V to -15V single ended).
Note that puts 30 watts of electrical power into the speaker. It does NOT generate 30 watts of sound energy - nothing like it. You probably get 0.1 Watts of sound energy, and 29.9Watts of heat out of it. The speaker coil has to be designed to get rid of that 29.9W of heat or it'll melt. It's this melting of the coil that limits speaker power, and is why concentrating on 20W, 30W, 100W or 250W is pointless. What you really need to know how efficiently the speaker converts the electrical energy into sound energy. A 10W speaker can be much louder than a 100W speaker if it's more than 10 times as efficient.
Speaker efficiency can range from 85db (crap) to 105db (pretty good). Doesn't seem much difference, but each 3db increase is a doubling in sound output, so 105db is more than 100 times louder than 85db - for exactly the same input power.
So all that 20W, 30W, 100W or 250W tells you is how hot everything is going to get, not how loud it's going to sound.
A piezo speaker doesn't have a wire coil, it works on stressing a piezo (sort of glass/ceramic) in an electric field. It doesn't have a fixed 'resistance' and behaves reactively. This can upset an amplifier designed to drive a normal moving coil speaker (which does behave resistively over most of it's useful range)