There is a bug in X3 (the game engine itself) that mistakes Discoverers for M4s. This means that they'll act as M4s in a task force, and will automatically set up a 3 ship 'attack force' when they see enemies. At least that's how I have understood it from Myros' descriptions earlier.
So M4s (and Discoverers) are pretty useless in the game, and in a task force. I recommend keeping all M5s until several of the pilots reach commander rank, so that you can put at least 3 M3s in a task force, or preferably all (with a few M5s as scouts). With only M5s in the force, they will not attack anyone.
I still haven't lost a ship. I'm running 3 task forces, each lead by a captain in an M3, and each with 18 M5s under their command. The two task forces in Kingdom's End and Home of Light have 18 Split Jaguar Raiders each (mixed in with a few Jaguar Vanguards), and I've recently set up the third TF in Antigone Memorial which has 18 Teladi Harrier Sentinels.
To be honest I also keep a sector defender in Home of Light when I have it available (a Split Elephant), since it's frequented by a bit more pirates than the other sectors.
The Split and Teladi M5s are the only ones that can carry APACSs, which makes a big difference with the way OOS combat is calculated (all numbers).
Finally, I haven't tried this on combat pilots myself yet, but there is a new script just released: The
Camouflage Device. It can also be bought in TerraCorp HQ, so it goes hand in hand with this...

When installed and activated, no enemy ships will attack it. Could be handy for keeping those trainees safe until they get bigger guns!
