The head and cab are a perfect match. The cab is 4 ohms, and the amp works with a 4 ohm load. If you add an 8 ohm cab to the 4 ohm cab, the total load will be 2.67 ohms--and your amp will burn up trying to do that. If you want, you can use the 8 ohm cab by itself, that's not problem; or you can use two 8 ohm cabs, which will make a 4 ohm total load.
You need to read a couple of things: first, the FAQ in the Amps forum. Second, there are several threads about combining a 4x10 with a 1x15; in fact I think one of those threads is on the first or second page of the amps forum right now. Oh yeah, and third: the FAQ in the Amps forum.