I would contend that Soldiers humping their own water puts even more necessity on reducing weight elsewhere. Water weighs what it weighs and that's not going to change anytime soon, so we can't reduce that. But if we can reduce weight elsewhere, then that makes for a more combat effective Soldier.
But the M4 isn't a PDW concept. Sure, a tank or Bradley crew is much better off with an M4 vs an M16, but that doesn't make it PDW. It's still very effective as a primary weapon system.
That's the thing though. Using the M4 isn't preparing for the last war. A linear and conventional battlefield doesn't automatically mean wide open expanses. The M4 would have been a formidable weapon in both theaters of WWII, probably in Korea, in Vietnam, in Panama and Grenada, in Somalia etc etc... The only area where it's not more effective than a full size rifle is in wide open terrain. But in those instances, Soldiers have crew served weapons, Mk48s and SDMs anyway. Each member of a fire team doesn't need to be able to reach out to 1000 yards. The fact is that most of the inhabited world is covered in either forests, farms, or urban/suburban terrain. And as time moves on, urban/suburban areas will only increase. The M4 is much better in all those areas than the M16.
I will say one caveat regarding future conflicts and the M4 still being better: The Arctic. The arctic circle will become a hot spot in the not too distant future(50 years?). Already, Russia is amassing new units for the arctic and rebuilding and constructing new infrastructure and facilities. The Scandinavian nations are responding in kind and starting to increase military funding for the arctic. Canada and the US seem to be a bit behind the curve, but we'll get there. That untapped and oil rich area has VERY murky borders which are still in contention. And the wide open expanses of that entire region would make it worthwhile to re institute battle rifles as the main weapon system should a serious conflict erupt there.
And I hate to burst bubbles, but the M14 really isn't a great battle rifle in modern times. For one thing, it doesn't hold its zero well at all. Though this may be improved with modern stocks and bedding. I think it's time to stop trying to put lipstick on a pig and retire those for good. It would be wonderful if they could be converted to semi-auto only and sold by the CMP. I would also love to see each squad equipped with one M110 or Scar 17 as an SDM rifle.