There are a few real disadvantages about running 4 ohm speakers that offset the power gain. Firstly, the effective damping factor will be halved, since the ratio of cable/amplifier output impedance to speaker impedance is 1/2 what it would be with an 8 ohm speaker. Secondly, cable gauge and length will be more of an issue since twice as much potential current flows into half the load impedance - the cable's resistance effects the circuit by a factor of two. Lastly, most current power amplifiers, even if "rated" into 4 ohms, do not perform as well into this load as into an 8 ohm load. The distortion is usually higher among other things.

A power rating of twice, i.e. 400 watts verses 200 watts will yield a 3db increase in potential dynamic range, assuming the speaker can actually handle it! Almost no current speakers can handle 100 watts of continuous power, let alone 300 watts - they will simply fail outright or warp their voice coil formers.

Another consideration is the fact that by far the highest power demands are in the bass region and if a subwoofer is used in conjunction with bass management, the mains do not need all that much power at all - even 100 watts in a typical home theater is more than enough in most cases.

This used to not be the case in the days of "stereo" - speakers then were much less efficient and were expected to reproduce the entire frequency range. Power amplifiers were only two channels and this allowed a higher current capacity per channel verses a 5 or 7 channel amplifier. You simply have only around 1800 watts available from your AC wall outlet too, and this used to be available for only two channels, not 5 or 7. Stereo power amplifiers used to be built with more output devices per channel, and this translated to more current delivery capability. As a gauge, a "perfect" amplifier should be able to double it's power output for each halving of load impedance. Thus, a 200 watt into 8 ohm amplifier would deliver 400 watts into 4 ohms, and some amplifiers of that era would even deliver 800 watts into 2 ohms.