Re: Ignoring Superman 3 and 4
It was most definitely a loose sequel and not a full on reboot. It would be like if Batman Begins used Danny Elfman's score, images of Jack Nicholson's Joker, direct quotes from Batman '89 ect.
It's pretty safe to assume Singer meant for you to watch the 1978 Superman before watching Returns. Returns doesn't really stand on its own. Imagine showing it to someone who's never even heard of Superman before and they'd be confused.