You have to keep in mind that Bethesda games are generally very poorly optimized, especially Oblivion. Even on my more powerful system I sometimes get dips into the low 40s and stuttering, even without mods. I can confirm, though that my old PC did run the game smoother at the same settings (max settings), albeit at a lower resolution. Oblivion is also known for having horrible multithreading support, another big cause for poor performance and stability issues.
There are a few mods that slightly help with those problems, but neither completely fixes them:
http://www.nexusmods.com/oblivion/mods/23208/?
http://www.nexusmods.com/oblivion/mods/45266/? (I don't know if there would be an improvement at all with just 4gb of ram, though)
There's also http://www.nexusmods.com/oblivion/mods/45749/?, but it makes quite a lot of changes I don't agree with and not all of them can be toggled off. Still might be worth a look.
Lastly, there's http://www.nexusmods.com/oblivion/mods/10400/?, but it doesn't really do anything you can't do by simply changing your graphics settings and is more of a nuisance than a help while playing. I can't recommend it, but that may be just me.
Enabling V-Sync also helped a bit, but only if the additional input lag is worth it. I simply hate screen tearing.