Can I use this to air down all four tires at once, or do I have to do them one at a time?

One at a time. The chuck connects to a single valve stem per cycle. On the trail, you'll disconnect and move to the next tire—takes a couple minutes total depending on pressure drop.

Does this pump require a compressor or tank to work?

The transfer function moves air between your vehicle's tires or from a portable tank if you bring one. It's not a powered compressor—it relies on pressure differential to move air, so you'll need either a pressurized source or a tire with higher PSI than the one you're filling.

Will this chuck fit my truck's valve stems, or do I need an adapter?

Shrader-style chuck threads directly onto factory Shrader valves found on most trucks and SUVs. No adapter needed. If your vehicle uses Presta stems, you'll need separate hardware.