M4+ Light Fighter

Perhaps the saddest joke of vanilla X3 for player-operated ships, M4+s were the aborted love-child of M3s and M4s. While it sounded good on paper (yet another incremental “in-between” step, has to be useful, right?), the fact was that M4+s had few advantages over their smaller brothers. They are, indeed, capable of mounting a handful of 25MJ shields, and with about double the shield recharge of the corresponding racial M4 (i.e. Boron Pike vs. Boron Mako) both in vanilla and XRM, but this came with a hefty decrease in speed in the olden days. Thanks to XRM, though, they have been improved enough to make them viable, but not enough to be overpowered; specifically, the speed reduction between corresponding M4/M4+s is not as drastic, and the weapon capacitor is beefed up. Cargo bays remain the same, though, namely that they are about the same capacity as their race’s M4.

As a Player's Ship
You might be better off staying in an M4 until you can get to M3 -- the loss of raw speed hurts most players, as player-controlled ships tend to benefit much more from the strategic use of thrust in combat situations.

As an AI carrier-launched fighter or wingman
Perhaps the only thing going for M4+s is that they have the same, if not slightly better, maneuverability than their little brothers, both in vanilla and XRM. Combine that with the increased shielding, and M4+s are best used as close-in carrier escorts -- you won’t be intercepting anything with them at range, but anything that comes nearby will face the wrath of a swarm of pesky M4+s, whose shields give them longevity and decent steering can keep them bearing down on targets, while their substantial energy banks will allow them to undock, begin spewing laser-y death across the battlefield, then redock as necessary to recharge more quickly (you ARE using a carrier command script/mod such as ADS and set the Docking Computer back to S-size, right?).