What is the real RMS power of the RZ4-3000D?

Power Acoustik rates it at 285W x 4 at 4 ohms, 375W x 4 at 2 ohms, and 750W x 2 bridged at 4 ohms. The 3,000W figure is max (peak) power, not continuous RMS.

Can it run four speakers and a sub?

It has four channels. You can run four speakers, or bridge two channels into one sub and use the other two for speakers. It cannot power four speakers and a sub at once.

Will it fit under a car seat?

The chassis is 12.25 inches long, 4.75 inches wide and 1.625 inches tall, so it fits many under-seat and side-panel spaces. Measure your mounting area first.

Yes. Each channel is rated for 2-ohm loads at 375W, and bridged channels are rated at 4 ohms.

Does it have a built-in crossover or DSP?

The listed specs cover power ratings and dimensions only. It is a basic four-channel amp, so expect simple gain and filter controls rather than a full DSP.

Drivers who want a slim, inexpensive four-channel amp to power door speakers or a bridged sub. Buyers wanting measured clean power should look at amps with verified RMS ratings.