Definitely plan on owning it, but I won't be getting rid of the 2.8. I, too, suffer from a bad back and an injured left wrist, so the lighter weight will be welcome, when it will do the job. But, it won't always do the job! I shoot theater and performing arts, and it can be quite dark in there. If you don't have to shoot in available darkness, the 4 is probably all you need.