I cut into strips and stored in plain white envelopes. I don't know that I'd recommend that, though, as it's easy to scratch the film. Once I wised up that they would do it with scanning, I'd take them in to the local lab to have them scanned and sleeved.
I have the same dilemma tonight! I processed my first 4x5's today. For the moment I have put them in separate (new) sandwich zip lock bags until I get some sleeves.