First, your power calculations seem to be off by a factor of 2. The actual calculation for an amplifier is far more complicated but using the equivalent DC will give worst case scenario. V^2/R = W for the first case gives 144/4 per channel giving 36 watts. Then divided by two gives 18 watts per speaker absolutely worst case. In reality it's less than that.
Same thing for the second calculation 121/7.2 gives 16.8 watts per channel.
Second, the link you provided shows that the amplifies are not 4ohm stable when using both channels in stereo.
Working voltage: 10~26V, Max output: 2 x 50W (left and right channels)/8ohm, single channel BTL power 100W/4ohm
These specs show that when using the chennels individually they are 8ohm stable. If you bridge the amplifier the it will sustain a single output that is 4ohm stable. (Bridging is only available on certain amplifiers, it is when the positive of one channel is connected to the negative of the other channel across a speaker. The amplifier is designed so that one channel drives the speaker normally through the positive terminal. The second channel drives the speaker with an inverted waveform but through the negative terminal.)
There are several problems with calculating amplifier power.
First, using DC for the calculation gives a wide ballpark. This is because the calculation is like connecting the input source directly across the speaker. In reality the speaker is driven by AC and will only see maximum input power at the greatest volume and for only the peak of a sine wave.
Second, sound is not a pure sine wave. It contains different frequencies and harmonics between 20Hz and 20kHz with varying amplitudes. This means that using calculations for RMS (pure sinewaves) is also not practical.
Third, speakers are an inductive load. This further complicates the power calculation because the speaker induces reactive power.
In reality they best bet is to match the RMS power of the amplifier as close as possible to the RMS power of the speaker. Then adjust the gain until you hear the speakers distort and turn the gain down a bit. If you can't match the amplifier and speakers closely a more powerful amplifier and weaker speaker is better.
In your comment you wonder if the amplifier would regulate the power down. They won't. I've just scratched at the list of what needs accounted for when preforming the calculation. There are many more that easily account for the dependency.