I would bridge two channels for the Sub, if possible, then use the other two for speakers. You don't need defining bass. Also, the human ear is incapable of determining Left & Right from frequencies below 100Hz, so a single SUB is sufficient. If you bridge, then you only need one sub, not a dual wound coil sub, just a regular single coiled sub.
Your amp is rated at 4ohms, so to get the maximum potential, you use 4ohm speakers. If you use 6ohm, you lose RMS and get more power converted to heat at the amp, and at 8ohms, you would cut that the RMS almost in half, basically. So, 100watts x 4ohm into 8om = 65-75 watts output, BASIC, not spot on, but you get it.
Now, if you use 2ohm speakers: 100 watts x 4ohm into 2ohm = 165-175+ Watts throughput, so to speak. You can place TWO speakers in Parallel on each channel to gain a 2ohm condition.
This is not a SPOT on conversion, but the higher the ohms at the speaker, the less wattage out the speaker you get when the amp is rates at lower ohm than the speaker. I don't think they make 8ohm car amps anymore, so amps today are either 2ohm or 4ohm, rarely 6ohm, and not 8 anymore.
Under powering really doesn't exist unless the speakers have a PRINTED minimum input wattage requirement, it will be on the back of the magnet, IF there is one. Speakers that require minimum wattages are the higher end 150+ watt speakers, and a lot are usually 6ohm or more impedance; and home speakers. You can't under power a speaker if you are using 4ohm speakers on a 4ohm amp, and there is no indicator that there IS a minimum wattage.
You definitely will not under power a 2ohm speaker on any 4ohm amplifier. There is a big difference in wattage in POWER watts -vs- RMS watts. A 100 watt amp my only produce a total of 50Watts RMS. I used to say RMS as Real Music Sound, but that's not what RMS means. I did that just to separate the POWER -vs RMS of the amp... Mosfet powered amps are the best.
The electrical power delivered to the loudspeaker and its sensitivity determines the sound power level generated, with the rest being converted to heat. RMS is more the actual sound level at the speaker, kind of... So, a 100 watt amp, with 50 watts RMS, you are losing 50 watts to heat generation, which is why the amps get hot. Amplifiers are limited in the electrical energy they can actually amplify, and on the other side; speakers are limited in the electrical energy they can convert to sound energy without distorting the audio signal or destroying themselves.
You would see a huge difference in say; two amps, Both 100 watts x 2. The CHEAP ASS brand will deliver 15 to 20 watts RMS to each speaker, while the good, Mosfet supplied amp will get you like 35 watts RMS each side. So, you then wonder why, TWO amps, both at 100 watts, one delivers less RMS than the other one. Well, with the cheaper transistors in the cheap amp, they are losing more power to heat, which is simply an inefficient amp. That can mean the AMP is not rated correctly either, but at least they tell you the RMS for the ohms used. It may say it's a CHEAP 4ohm amp, but in actuality it would work 100% better with 2ohm speakers.
A 100 watt amp should have at least a 10 AMP fuse. The cheaper one? May have a 7.5Amp fuse, telling you you, I'm all heat, no power... I have been hooking up car systems for 24 years, and no complaints or returns, I know how to get it done.
