I looked through the Laserdisc Database and it seems that apart from the two special LDs that had all over their contents ported to the Quadrilogy set there are only the theatrical trailers for the first 3 films as extras on the other releases (it's those trailers or no extras at all).

However there is a German set and a Japanese set that might be of interest. The Japanese set seems to have 5.1 AC3 surround mixes. Both the German and the Japanese set feature "The Making of Alien Resurrection". The japanese has in-picture subtitles while the German LD appears to have only German audio (but no subtitles).

Finally there was an LD for Alien that supposedly has "sound remastered from the 6-track magnetic masters prepared for the original 70mm theatrical prints."

I know we probably don't need any of the above, but they are the only sets with notable features aside from the Alien and Aliens LDs that the Quadrilogy set took content from.