It's possible that it's the problem, that they're running right to the wire on disc space--but comparing disc 3 to disc 4, I find that hard to believe. (I think both discs should have about the same amount of replacement material, while disc 4's featurette is actually shorter.) I'm also...
Though at least the Blu-rays are offering new stuff; we never got S2/S3 remastered in HD before (just S1), nor any of the pre-remastering in HD. That makes it an upgrade even for fussy old rustbuckets like I may resemble. ;)
It depends, also, on whether it's a flipper or single-sided. The single-sided were limited to three layers total, while the flippers could have two of each on a side. (Don't know if there was an initial single-layer "teething" period or not.)
Only four per disc? Ideally, but in the TV releases from WB they've been putting 3 episodes on single-layer discs or as many as 7 on a double-layered disc. (My only other TV release is Torchwood, which isn't exactly a typical American release case. ;))