I can't speak as to the 4-bag system personally, but I agree that it makes sense for it to be rated as a "survival" temperature rather than a "comfort" one.
On the other hand, if you're looking for a decent inexpensive top quilt or underquilt for down between 40* and 50*, take a look at poncho liners.
I've found that a poncho liner is an acceptable top quilt down to 50* for most folks without any supplementary insulation and 40* is quite reachable with normal supplemental clothing (long underwear, sleep socks, and an hat or balaclava). Me, I'm comfy pushing it to freezing in a T-shirt and shorts as long as I have adequate bottom insulation, but I'm an human furnace.
As an underquilt, well, I'll just point you at the two different versions of a PLUQ: no-sew
and sewn up
Poncho liners can be had for ~$10 to ~$30 a piece, depending on where you buy. It's hard to beat the weight versus cost ratio on 'em, though they (like all synthetics) are a bit bulky.
Hope it helps!