So, say another war starts. Do I, as a loyal fighter for my land, care whether the enemy soldiers are MP4 or MP5?
There should be a natural progression to MP5, especially now that MP4s have the opportunity to train their stats before advancing by lowering EXP.
If one MP4 advances alone, yes it's going to be difficult for them to get on their feet, but if two MP4s advance together, they've got training partners of somewhat equal level, and if three or four advance together, they've got a whole group. Then, when another group comes once that group gets stronger, they can attack these new MP5s while they train themselves for the next group.
There are so many reasons to disagree with this, the least of which is "Hurr durr I want to attack you".