We use ProLine Calibers as a control tire for the Dutch/Belgian Nationals (the ones on dirt and clay, that is), in M3 and M4 compound.
I found that the grip levels and tire wear were more influenced by weather conditions than the compound. When the clay is a bit moist there is loads of grip and literally no tire wear, they'll look new when you wash them off at the end of a meeting! In truly dry conditions they tend to wear down much quicker, at the end of one race day you'll have a training tire left, at most - Holeshots will wear even quicker than that.
The M4 compound will help to find more grip, but will wear slightly quicker. Until recently there was no local supplier that offered M4 Calibers for a reasonable price (they used to cost 5GBP a set more than M3s). That has changed, so though now I do drive M4s, I just want to mention I've done just fine with M3s in the one and a half years before that
Only go for M4s if you don't have to spend (a lot of) extra money.
The standard insert works fine to start with. I would try a standard set back to back with a set with the standard inserts cut to the width of the tire. A TLR Team Driver said he never looked back after trying it. He then let me try a set on my DEX210: The car seemed to be unnerved less and a little less bouncy (particularly in sideways motion). I'll be trying it more extensively next (outdoor) season
The 4WD front tire insert is the same as a rear insert I recall, so please check with 4WD drivers if they need to be narrowed to get enough steering.
Also worth noting by the way (for 2WD), it's not essential to run the same compound front and rear: For example, 4-Rib M4 tires only hit the shelves recently, before that the choice of tire here was 4-Rib M3s or even a different brand