There should be a 16 ohm tap on the output transformer that's not wired to the output jacks. It's a pretty simple switch to make the outputs 8 & 16 ohms instead of 4 & 8 ohms. Even easier; odds are that cab has 16 ohm speakers wired series/parallel, and it's pretty easy to rewire it parallel/parallel to get a 4 ohm cabinet impedance. Plenty of bass players use/have used 4X12's, it depends if the drivers in that particular cabinet are up to the task.
Some guitar speakers won't hold up to sustained bass guitar use, or sound very good doing it, and a typical sealed 4X12 won't have nearly enough bottom end response to suit most bass players.
I guess the most important thing is if you like the way it sounds!