Certified Def's come out about every 4-5 hours, daily. They are about 40+MB in size... I like to keep servers right around that schedule, up to 6 hours per liveupdate.
This seems like normal behavior, no?
I would most def, upgrade the SEPM to MR4 MP2. Watch for a few weeks, then work on rolling out client upgrades seeing as how you are a major MR behind anyways... ;-)
NOTE: MR4 can manage MR3 machines no prob :-)