In general terms of quality, should a 26 episode series be put on 3 or 4 Discs?

[tsunami]

4 discs sounds better but I would only say that if the encoding job could match the amount of space given. Decent video bit-rates should be around 4-5mbps and the 3 disc 26 ep usually are from 2.8-3.7mbps range so that is a 1/4 compression rate. I've never seen a 4 disc sets video rate but I assume they aim for 3.5-4.4mbps since they eliminate a few eps a disc. This is a hit and miss thing not exactly a fact dictated thing.