So I've been messing around with Mx4 Storm model from BF4
And it looks like this could be made as custom weapon albeit with some (pretty major) downsides.

So we have 2 weapon base to use for Mx4:

- Sub2000 (Cavity 9mm) :

• weapon handling fits pretty well, but equip anim with its flipped motion isnt;
  
• full reload anim isnt, probably will disable it;
  
• can only have semi auto firing
  

and - MP7 (SpecOps SMG) : 

• weapon handling will have vert grip that's too close to pistol grip; 
  
• full reload anim is acceptable but mag reload anim will look weird; 
  
• can have full auto firing
  

If we're using Sub2000 base, it'll have moderately high damage with high ammo capacity. Essentially making this a Cx4 Storm variant.
If we're using MP7 base, it'll have similar stat to MP7 itself. Mx4 variant.

Note this is all theoretical at this point, and if late in the custom weapon making, I deemed it too bad/ugly/horrid as custom weapon mod, I might not release it at all :\