Pump/Semi toggle could be an interesting feature, but to make it relevant at all, both modes need to have their strengths, and they probably need to use the same pool of ammo, since unloading a tube mag to then refill it with something else would be pretty slow.
Benelli M3 also has a pump/semi toggle, but SPAS12 would be better than that for KF2, more distinct from the M4's looks.
Making pump mode more powerful while shooting the same ammo as semi mode is a straightforward approach, and could probably make for balanced and fun gameplay, letting the player always reconsider whether they want damage per second or damage per ammo. But it's quite a bit less realistic than KF2's usual standards of leaving the magical bonus damage to be done by perks.
A more realistic option would be to represent inconsistent ammo, so semi mode malfunctions occasionally causing a disruptive delay, but usually has a high rate of fire, while pump mode has a consistent and moderate rate of fire. This could be balanced, and it would be very unique among KF2's otherwise reliable guns, but lots of players probably wouldn't find that to be very fun.