Even with the 18" depth and the 4' length, I don't think I would put any more than a single pair of discus in that tank. They're fairly large fish and the tank is going to seemed crammed with any more. I like to think the fish in my tanks have lots of swimming room. The fish may or may not care, but I've never like the looks of tanks that are loaded to the gills (pun intended). To me an aquarium is more than just a box to keep your fish in. Huge numbers of large fish in a tank thats 4 feet by 2 feet just doesn't look realistic or comfortable to me.
That being said, I definately tend to understock my tanks. It's not that I don't think the filtration could handle a higher bioload, but I just don't like a crowded look to my tank. Stocking lighter will always leave room for impulse buys, rescue fish, unplanned fry, etc. The stock you are moving over from your 20g is more than what I keep in my 65g.