I'm debating between buying these two systems. I rpeviewed each on collagevideos, and it seems that both offer what I'm looking for: a solid workout with not too much jumping around (so I don't disturb the people beneath me). I've pondered getting the Firm for years, and I need to decide between these two systems. I don't really want to shell out the extra $$ for the transfirmer, but if it's much better than the fanny lifter box, please lemme know! Thanks

I have BSS1, and not BSS$, but I have used the TF at The Firm studio in Columbia, so I can give you my opinion on the equipment and the BSS1 workouts.
What I like about the Fanny Lifter (FL) is it is small, the weight holds 300 pounds, push-ups on the FL done on the FL at a diagonal, can be used with ALL pre Goodtimes Firm workouts as well. What I like about the Transfirmer (TF) the incline really helps lunges be much easier on the knee joint.
What I do not like about the TF, is the weight limint is 275 pounds, incline aerobics feel very awkward on the ankle joints, though it does hit the glutes and creates a calf stretch when being used, no diagonal pushups. Takes up a lot of space compared to the FL.
As for the workouts themselves, I have not seen the BSS4, BSS1 seems very scripted, some of the leads appear to be uncomfortable in front of the camera. I LOVE Body Sculpt led by Jen Carman though as it is 30 minutes and divided into 2- 15 minute segments. Her classes were very tough and challenging and this w/o is similar to a live class with her.
I have read tons of great reviews for the BSS4 and most people choose this system over the thers that have come out.
If you choose to get the BSS1 system, there are 6 add on wo's and you can get the BSS2 system which also uses the FL (I know there is a sculpting stick used as well, you can modify with a dowel/broom/mop/body bar/) and all of the old Firms use the smaller box.
Should you choose the BSS4 system, you can also do the BSS3 system as this system also used an incline box (not as sturdy as the TF at all, the Box was determined to be unsteady and a good piece of trash by several people who had it.) It could also be used with other workouts, but you would have to modify the moves created for the small box to the long box. It would not be possible to have the FL and later expect to do the BSS4 (or BSS3) workouts since it does not incline.
If you'd like a link for more info on the BSS systems, pm me.
Hope this helps!

Thanks for such a detaioled reply! I think I'm going to go with the Fanny Lifter box for some of the reasons that you mentioned i.e. it's smaller. I read all the negative press about the BSS3 so that wasn't in the running. But I like that I can do the BSS2 dvds and older dvds with the fanny lifter. I broke my ankle almost 2 years ago so if the incline on the transfirmer is gonna aggrevate that (and the incline is the major difference between the fanny lifter and the transfirmer box) then I'll stick with the fanny lifter.

I have read that some of the BSS1's are like $8 at Ross, but I don;t know if those were alone or with the FL.
If you already have a normal step, you can substitute a step stool for the FL in the workouts (of course they don't show this anymore though since it has become commercialized)
Since you metntioned the jumping around bit, you might also like the Fitprime sereis that uses the Tallbox (aka FL- and they show the substitue in the videos for the equipment.) These workouts were created by the sisters that started The Firm. They are very joint friendly and the hips/core/lower ab area gets attacked like I have seen in no other workout! you can view clips of them at www.whfn.com.