What is the difference between the 2-wire and 4-wire versions?

The 2-wire version has a single hot and ground lead, so it drops into standard humbucker wiring. The 4-wire version exposes both coils' leads, letting you coil-split or wire series and parallel.

Is this a set or a single pickup?

It is one bridge-position humbucker. A neck pickup is sold separately if you want a matched pair.

Will it fit my guitar?

It is a standard humbucker size, so it fits guitars routed for humbuckers, such as Les Paul-style models. Check your cavity depth and mounting ring before buying.

Only the 4-wire version can be split. The 2-wire version cannot be split without modifying the pickup.

What does it sound like?

Lambertones voices the Crema as a warm, vintage-leaning bridge humbucker. Published output and magnet specs are limited, so judge it by the maker's description and player reports.