Ok, my opinion, after watching this SR like 10 times.
I think all the previous movies fit into the continuity (that ALL).
STM and SII were the films that this film was supposedly supposed to be a sequal to. However, after watching SR a couple times, most of SIII and SIV seem to line up with SR aswell.
SIII could have happened over the space of a week. And in theory, could have occured only a couple of days after Luthor tried to Nuke California and the invasion of Zod. I always looked at SIII as 'a week in the life of superman'. A short vacation to Smallville, after having a megabattle with Zod and co. A well deserved vacation.
SIV, much like SIII could potentially have taken place only a few months after SII. The nuclear theme of test launches in both STM, SII (the eiffle tower) and SIV line up nicely and could be looked at as the same run of missile tests. It could have been Superman's arrest of Luthor, and his failure to stop the missile crisis, which cause's him to think about krypton and leave, neglecting to remain in order to testify against Luthor.
There is only 2 facts from the Reeve movies that dispute this theory.
1) in SIV, injured superman uses the last of krypton's energy to heal himself to rejoin the battle against nuclear man.
3) Ma Kent had died by the beginning of SIV and Clark was trying to sell the farm.
Anyhow, I think all of the previous movies pretty much line up with SR, without causing to much continuity problems. Comments, opinions?
PS. Smallville also fits into the continuity so far aswell. Nothing in Smallville has defined it yet as a seperate universe (so far atleast). Smallville is just a more detailed look at Superman's youth, and the '12 years' he spent training at the fortress (who says Clark stayed there the whole time listening to Jor'el lecture the encyclopedia galactica?).
Other than the 2 examples listed above, I can't seem to think of any other instances that would dispell continuity between these various properties. As Singer said very long ago, he's trying not to step on any toes with SR. I also think that Smallville is behaving in the same manner. They are trying not to step on any toes either. I think it all works out pretty well.