Simple one first. A "perfect" amplifier will deliver twice the power into half the load impedance. So 100W at 8 ohms = 200W at 4 ohms.
Problem is that "perfect" amps are few and far between, so 160W is a safe bet into 4 ohms for a 100W into 8 ohms amplifier. They do not publish this spec, but I did find it on another thread for the amp. Make sure you set the amp to 8 ohms, not 4 ohms, even with your 4 ohm speakers. Why? This is an American legislation thing that reduce the voltage rail on the 4 ohms setting, so the amplifier will be louder in the 8 ohm setting. It is less likely to cut out as well.
Turn on the soft clipping, as this will prevent damage to the speakers and output devices in peak conditions.
In terms of the phase reverse, this is trickier than it is on a pro amp, as you cannot just flip the phase on the input and swap the output wiring. You will need some electronics to insert a phase reverse and I doubt you will find a box off the shelf, even though it is very basic electronics. Just a pair of op-amps designed with unity gain, with one wired "hot" and the other "cold". About a dozen components, a simple power supply and a box to put it in.
The down side is that a 15% increase in power - if achievable, would be barely audible in terms of usable volume. Worth saying that in 20+ years of pro-audio engineering, I've never done this, nor seen anyone else do it either, so probably no real savings. You might even find some m-fers do this in any case and just don't tell you!