Here's a possible idea, but it involves modifying the weapon entry itself, and it may not work properly. Set up your 4 firepoints, and have three slots on the turret's entry. Give slots one and two individual weapons, but for turret three use a weapon with a swarm of 2 and no inter-swarm fire-delay. Then use the "fixed firepoints" subsystem flag. Now, this will definitely cause the weapons to stick to their firepoints, but with firepoint 4 being unassigned, I'm not sure if it will allow the swarm weapon to use it. It may result in firepoint 4 not being used at all. Really, there needs to be some SCP upgrades to overcome this kind of situation.
Now, if you want mixed weapon types, like combining pulse or beams with missiles, then its a synch and has been possible since FS1. Just look at the Faustus table entry. You can assign up to 3 laser/beam and 3 missile slots per turret. There's still the limit of 4 firepoints per turret, but you can get around that in the model file by making two turret entries on the same turret submodel with 3 firepoints each... which was done with the original Aeolus model for its flankside AAA and Flak turrets, though without any multi-slot or mixed weapon types.
It may work for you.
Later!
