Rail systems do not add much, if any, weight to an M4. With one installed an M4 becomes more versatile and basically has the same weight.
The key here is material. Any accessories made of aluminum and synthetics will add minimal weight to a weapon. In previous eras, the majority of arms were made of steel and had wood furniture. An M16 or M4 keeps it's steel content to the barest minimum and has no wood furniture. This in itself saves a great deal of weight. M16's are far lighter compared to the BAR, Garand, M1 carbine, and Thompson subguns. Accessories for the M16 family of riflesand carbines also have to be made of the lightest materials possible.
A vertical foregrip can aid in maneuverability in close quarters. A flashlight is invaluable in the dark for target ID. A reflex sight makes target acquisition in close quarters much, much faster. A laser is a must on a rifle if using a gasmask, an IR laser if using night vision.
Are all these absolutely necessary, maybe, maybe not. The thing is, most infantry and support personnel won't attach or carry these itmes. Most units use stock M16's, with at the most a small scope.
The pictures that we see the most of these days are of special operations and special warfare operators who specialize in close quarters engatgements. Most of them feel it is necessary to have these items because it gives you a slight advantage in combat. Any advantage is better than none. 99% of the time they will mount some of these accessories. Sometimes they will not. They can choose if they need these itmes and the extra weight or not.
Most times these guys won't really have to hump it that far or that fast. Even if they do, they are not (at least they shouldn't be) humping it with a 50 lb. pack on their backs. Go fast guys travel light. So most of them figure one or two pounds on the weapon is not that big of a deal because they aren't carrying extra weight on their backs.
An ideal loadout for an operator is very light. A weapon, a sidearm, a knife, extra mags, water, communications gear, spare batteries for equipment, maybe a backup sidearm and knife and light, and an MRE if he feels it absolutely necessary to carry one. There shouldn't be that much more than that.
A medic/corpsman will carry a large/medium first aid pack. The unit might carry explosives if it is a demolitions mission. Restraints would be carried if they plan on taking prisoners. It really wouldn't be that heavy a loadout compared to your average infantry unit.