I am in need of zimmerit for this kit. Will the early or late set work?

The shape and dimensions of the areas covered by Zimmerit did not change over the course of production. In any case, early, middle, and late are terms invented postwar--Krupp Gruson only ever built one variant of the Sturmgeschutz IV, officially, which had cumulative detail improvements over the production run.

I have seen sets for the early and late stug iv, but not the mid. What is the difference?
btw: I don't know why I said "G", when it should just be the mid production.

Original production versions had the same superstructure as contemporary StuG III (except for the driver's cab), so they had the folding shield for the radio man's machine gun mount. The size and shape of the driver's hatch soon changed from a simple rounded rectangle to a slightly larger, more complicated shape. Very early machines might possibly have had the dome-shaped hubcaps on the road wheels seen on Panzer IV G and early H models.
Later, the machine gun mount was changed to the rotating, remote control type, as seen on the Hetzer, and the split hatches were changed to open sideways instead of front to back. Around this time, a folding gun travel lock became available for the glacis plate. The raised cover over the radiator filler was changed from having sloped sides to vertical sides (also seen on Panzer IV J). Extended covers were fitted to the driver's periscopes to shield them better from glare and rain. Near the end, the schurzen supports were changed to allow the plates to swivel if they struck something, rather than being knocked or torn off.
Zimmerit use ended in September, 1944, so your vehicle would not have most of these late features.
The point is, these changes were added piecemeal to production. There was no official point when all the new features suddenly appeared, and old features could reappear if stocks of old parts were turned up in Krupp Gruson's warehouses. Best bet is to model a particular machine, if you find a good photo or two.