Mixing 5.1 into 4.0 in Real Time

A Dolby digital pre amp should be able to do what you need. Although that's likely to cost more than a used receiver.

Technics made a unit (SH-AC500D, I'm not sure if the older/lesser model (SH-AC300) would also work) a few years ago that will take 5.1 and output 4.0 (or any other number of configurations), but it won't do that with DTS signals, Dolby only, so if you're only interested in AC-3, I'd give that a look. They were only a few hundred dollars brand new, and often go for a hundred or less in decent used condition (you'll want the remote, as it has a few buttons not available from the front panel).

Just configure the unit to only have front and surround speakers, set them up as large/small depending on what suits your equipment, turn the subwoofer and center off, and feed your Dolby 5.1 signal into the unit.

I know Yamaha made two processors (DDP-1 and DDP-2) that's relatively similar to the AC-500D, at least on paper. I don't know if they will re-map to 4.0 though. You might at least keep an eye out.

Finally, I've seen some pre-amp/processor devices from Marantz, Accuphase, and McIntosh that look physically similar to the Technics and Yamaha units, but I've never gotten beyond seeing them in a few sparse pictures here and there, so for all I know they're incredibly expensive or entirely unsuitable (or both). There are also brand new components available on ebay, and I always tend to forget the model numbers when I need them. They're usually around $130, look for a Dolby Digital Decoder.

For the ~$100 you'll spend on a new processor, you may want to consider just looking for a used receiver that will replace the quad receiver (unless you use the quad feature for quad material) - you may get lucky there. Alternately, save the money and purchase a new receiver in the $400-$600 range that will decode Dolby (and probably a lot of other things) into 4.0 or whatever other configuration you want (so you could expand from quad at some point, in theory).