Hmmm, some points about weight.
The M4 weighs 7.8 lbs; the Remington 870 Express Tactical pump weighs 7.5 lbs - less than 4% lighter than the M4. A typical AR carbine weighs about 6.85 lbs unless it's a piston version which puts the weight up to about 7.3 lbs. However, a Benelli M2 only weighs about 6.7 lbs, so it is lighter than the M4 but only by about 13%.
870's with an extended mag tube, sidesaddle, and forend with light, e.g. Wilson Combat's Standard tactical shotgun, weighs in at about 8.6 lbs and that's pretty much the standard setup for a tactical shotgun and that weight is pretty well accepted.
The value of weight is softer recoil. I have an M2 Tactical (6.7 lbs) and it has a harsh, hard recoil. I know, because I shot 20 rounds of low recoil 00 and 10 rounds of slugs through it and had a sore shoulder for a day or so - and I know how to mount and shoot a shotgun.
Then I took my 870 tactical, essentially a Wilson Combat Standard model, about 8.6 lbs and shot the same 20 and 10 rounds through it as I did through the M2 and then 25 more rounds of #7.5 shot after that for a total of 55 rounds - the recoil was mild - no sore shoulder.
Weight is a two-edged sword. Heavier is more to carry of course, but easier on the shoulder and faster on follow-up shots. Lighter is less to carry, but hard on the shoulder and is slower on followup shots.
So if you carry little and shoot a lot, like I do, the heavier is a definite advantage. If you carry a shotgun a lot and don't shoot it much, then lighter might be better.
I'll be shooting my Benelli M4 for the first time today and report back what I find.
Plus, the M4 is a bit more than just heavy and expensive. I believe it is parkerized and hardened in some other ways that I can't recall now. Also, and I'll have to confirm this, I believe M4's will cycle lighter loads than M2's. I base this on the fact that the Benelli spec sheet for the M2 has a minimum recommended load, and the M4 does not. Of course that's only an indicator and doesn't mean the M4 will shoot a lighter load. But again, the heavier weight of the M4 should help it shoot a lighter load.
The M2 tactical and M4 both come stock as 5+1 guns and extension tubes are available for both, plus unique quick detach side saddles with capacities of 4, 5, 6, & 8 rounds, are available for the M4. And full length rails are available for M4s.