Do these subwoofers come in a box?

No. They are bare drivers only. You need to build or buy an enclosure and wire them yourself.

What amplifier do I need for four of them?

Each driver is 4 ohms and handles 125W RMS, so four together need a total of about 500W RMS. Match an amp that stays stable at the impedance your wiring creates.

Can I run just one or two instead of all four?

Yes. Each subwoofer works on its own at 4 ohms, so you can use one, two or all four depending on your amp and box.

Are these good for a boat?

They can work in a boat if mounted in a sealed enclosure away from direct water exposure. They are standard car-audio drivers, not marine-rated, so protect them from moisture.

How do they compare with a single 12-inch sub?

Four 8s have more total cone area and can play louder across a wider area, but they need a bigger box and more wiring. A single 12 is simpler and hits lower with less space.

What size enclosure do they need?

HiFonics does not publish a recommended box volume here. Ask the seller or use a box calculator for 8-inch 4-ohm subs, and keep each driver in its own sealed or ported chamber.