We asked the original RF1 level design lead about it. It's been a long time, and his memory is a little hazy, but he believes that it was an acronym for Standing Room Only.
The SR3 Trouble with Clones DLC had superpowers that were well received by community. We planned to do another DLC that expanded on them, but old THQ decided that we should scrap our original plans for SR4 and expand that SR3 DLC into a full game instead.
youtube.com/watch?v=bXKeOTo9…
Nothing is ever set in stone and planned ahead that far. A sequel looks at the successes and failures of the previous game and the story and gameplay are adapted accordingly. Sometimes the story even drastically changes partway through development if things aren't working.