Shockwave at Great America also had 4; 2 red and 2 yellow (one later painted blue). Six Flags really pushed capacity back in the day; especially cared about customer service and didn't have pay-to-cut. Magic Mountain always had extra trains on hand; Revolution had 6 and ran 5. Colossus was built and intended to run 10 (!?!) but only ended up ever running 6 at a time.
Related... at Great America, we had 3 on Demon - ran with out stacking. American Eagle had 6 until a minor collision in 1985 (it was nice to always see 2 trains on the lift). Whizzer ran up to 5, and still has 5 (rotating them in and out through awesome full-rehabs every year; there were 3 on the track this November). I've always said Great America was the one hold-out for at least trying for good ops.
Viper could have run 4, but Arrow sometimes designed very short blocks and brake-runs. Like Tennesee Tornado; they only run one because stopping a second on that stupid-short brake run "tears everything up."
One more thing on Viper, Love it! Happy it's still here - without those wretched rubber vests (I know some people like those). Rode Viper in May, during the 3 hours I could stand at Magic Mountain. Mostly there for X2, Revolution and Ninja. I like old sh*t