Wow those are some good pointers! Thanks to everyone for sharing their scenarios. I'll definitely take a look at how much overlap there are between schools' rec forms... (for the ones that I have access to, ie. not mailed directly to the reference) if they're quite similar then I won't feel as guilty having my supervisor do all 4.

As you guys mentioned maybe I'll even come up with a set of template points and give them to my references, it'll probably ease their workload by a bit.
By the way I just spoke to my supervisor to give her a heads-up on my intention to apply to business school. Didn't get exactly the same reaction as I had hoped.

Her reaction was basically that I should definitely aim for an MBA for attaining the credibility and maturity, but only to part-time programs, and also only when I have a few more years of exp under my belt. If I matriculate next fall I'll have 3 years of exp + another 16 months internship exp, so I don't think I'm really sub-par in that aspect. I think her reaction was due to the fact that no one from our dept has left to pursue a full-time MBA before, so she wasn't so sure what to make of it and whether it's the right thing for me to do. Anyways I didn't mention the reference thing since she didn't seem to be fully-supportive as of yet. I think I have to do more convincing and really ease her into it...