The Bassilisk.

It has four mountpoints, one artillery cannon, long ranged (and "bolted" in, can only move up and down without the vehicle).
It also has one HMG on the side (movable, but not completely), and of course, it shouldn't have as much detail as the picture suggests, but still should have a "slit" for the driver, and a rear platform for the cannon gunner and unarmed rider. If you wish, it can only be 3 mountpoints due to the uselessness of a second cannon rider. It can also be fairly slow, seeing as it's an artillery tank.
Note: I didn't make that picture.