I think your A/B split is a great idea. Going to the 4 day split and adding the assistance lifts will increase your workload without making each session super long. Also, it will change up what you have been doing, but not in a super radical way.
I would suggest having "Heavy A" day, "Heavy B" day as well as a "Light A" and "Light B" day.
On heavy days, do 8 sets of 3. Start light, do a triple, then add weight, working up to a top set of 3.
On light days, do 3 sets of 8(after warm-ups), with the same weight for 3 work sets.
This would keep the same number of lifts per session, but you'd be using way different rep ranges and weights than you've grown used to. Again, changing things up without making any drastic changes.
Try this for 5-8 weeks and see how you like it. If you like the split idea, and you like doing assistance lifts you can progress to an upper/lower, or 5/3/1, or Push/Pull/Squat, or even some kind of bodybuilding routine.
If you decide you want to go back to a full body routine you can try the Texas Method, Madcow, or a Bill Starr layout. These are all full body routines that use a heavy/light/medium layout, instead of grinding away at the same weights day after day.
Or, if you really like doing tons of non-max sets in a variety of lifts you can try out an Anthony Ditillo program.