A 4.5m mech is still too large. For it to be of real combat value you'd want it to be no larger than a person. Think of the armor that the Master Chief wears in Halo. That's about the upper limit, maybe a little bigger for something that's more logistics focused(think man sized forklift, aka powerloader). The strength of an exoskeleton is that it makes a human sized target require anti-vehicle weaponry to take down. Anything bigger than a person, with the type of armoring limitations that would hit a power armor as it increased in size(basically the surface area would make the damn thing impossible to armor adequately, and would make it FAAR to heavy), would make them dead meat to tanks, aircraft, and other vehicles.
The advantage in combat of a functional power armor, is that it negates the primary weapons that insurgents/3rd world armies have access to(heck even typical modern infantry). With the prototype Sarcos suit, assuming it's militarized and it's capabilities are a little better, it would be almost impossible to take down with conventional small arms(e.g. assault rifles). Basically to drop one of the exoskeletons, you'd need like a .50 caliber weapons system, and that's simply not man portable. You can't arm an insurgency with .50 caliber weapons(and no a desert eagle pistol does not a Ma Deuce make), and RPG's can't be fired from inside of buildings (the backblast would kill the guy shooting them, to use an RPG you need to be outside or maybe a very large room) and aren't accurate enough to hit man sized targets easily.
This means that the AK-47 which is the predominant equalizer at close ranges(it's almost as good as an M-16, and with decent training can be VERY effective), is no longer that useful. The reason this is important is because for 50 years the AK has been the great balancer of armies. It was cheap, effective, and just about as good as the best rifles in modern armies. With the exoskeleton, all of the sudden insurgencies need to have some major budgets to acquire the weapon systems to try to combat exoskeletons, and short of being mechanized, it's impossible to really transport those systems. This means that for the first time insurgent infantry can't fight toe to toe with our infantry, because the weapons available to them can't do the job easily.
Basically it comes down to this.
Infantry Pros
Extremely mobile(tactically, not strategically), cheap, logistically easy to support(only need some food and water), fantastic situational awareness, versatile(can perform many tasks, from combat to building bridges), can take advantage of terrain the best out of all types of units(can go places other things simply CANNOT go), available in extremely large numbers.
Infantry Cons
Easy to kill(needs to be said), not mobile strategically(in other words they can't move a 100 miles per day), firepower limited to what they can carry easily.
Exoskeleton Pros
Potentially more strategically mobile than infantry(the armor is self supporting so the soldiers won't tire so assuming they've got adequate fuel, they can travel longer distances on foot), bigger guns(could possibly wield small vehicle weapons or crew weapons like .50 caliber machine guns, .30 caliber machine guns, or grenade launchers. However anything bigger would require a "real vehicle"), harder to kill(wearing a 150 lbs or ceramic armor and kevlar means it'll take some elephant guns to drop these guys), can go nearly everywhere infantry can(almost the same size), situational awareness almost as good as infantry(hard to see what's going on outside of a tank when the hatches are shut), psychologically intimidating(seeing an exoskeleton tear doors from hinges would scare the bejeezus out of an insurgent), potentially very useful logistically(carrying artillery shells, unloading trucks, etc.), possibly very accurate targeting/enhanced capabilities(computer assisted weapon control systems combined with a hydraulically damped body means = accurate shooting).
Exoskeleton Cons
More of a logistical burden than infantry, have some problems associated with vehicles(fuel, parts, maintenance, etc.), strategically harder to transport(in other words, a soldier who normally weighs 220 lbs fully loaded with gear, now weighs like 500-600 lbs), expensive(I estimate it'd probably cost 500k-1 million to outfit a soldier with some battle armor like this), smaller numbers(for the reasons of cost. I could see perhaps 1 guy in a platoon being equipped like this, and perhaps having specialized heavy assault squads of exoskeletons used for specific purposes).
Exoskeletons wouldn't replace infantry by any means, but in certain roles they'd be dominant(especially urban assault). In jungles, swamps, and aquatic environments though infantry would still be predominate(the weight of the exoskeleton would bog down in those places). However exoskeletons would be just as vulnerable to vehicles heavy weapons as infantry, so in open terrain I'd rather be in a tank or a Bradley, and they'd cost almost as much. So in the end it's sort of a trade off since exoskeletons are a kind of trade off between vehicles and infantry.
