But they keep the 4-speed transmission? For what purpose?

Presumably so it's an easier conversion - you replace the motor but don't have to replace the rest of the drivetrain, and maybe you want the gearstick inside for the look of the thing (although I imagine you likely wouldn't have to use it most of the time).

Probably to interface with existing cars by replacing the engine, retaining everything after, including the existing transmission, differential(s), and suspension.

This is a straightforward way of electric conversion. You plug in the motor in the place where torque is expected and let the rest do its job.

Gearboxes in EV conversions are usually locked in the highest gear with clutch and lever removed.

Reduction ratios, greater comparability due to interfacing links, attachment points, just a few guesses. Most ev conversions I've seen keep the gearbox.