Obviously, you cannot record at 120 FPS on older devices due to hardware limitations, but with the tweak, you can record at 60 FPS, up from the standard 30 FPS. Therefore the slow motion will not be as smooth as the iPhone 5s.
Enable iPhone 5s Slo-mo video capture for unsupported devices and included options for:
● Set the slo-mo framerate.
● Force Slo-mo editing for all videos.
● Set Slo-mo rate. (slow factor)
● Adjust the slow part of the Slo-mo video volume.
Important Note: Tested in iPhone 4S and iPad 2, the maximum slo-mo framerate is up to 30 fps only, else at 60 fps and may not more than this. Also, incorrectly framerate setting causes the camera app to crash.
Currently the tweak is a bit buggy, so you might want to hold off for now, however some users are reporting no major issue with it.
You can download Slo-mo Mod, which requires iOS 7, from the BigBoss repo for free.