By getting an M4 style gun, you are already selecting something that designed to be a smaller more compact version of a larger gun. I actually just purchased this rifle from a FilForce player too. Small world. Consider choosing a gun designed to be bigger, such as an M16. Although, it's generally accepted as dumb to put a collapsible stock on a full size weapon. The crane was created to facilitate different size people and operational locations with it's adjustable length of pull.
So:
1. Not even "real steel" stocks and parts on the M4/M16 are made of metal. They have quite a bit of "plastic" in various sturdy forms. You could find a large weapon you prefer and then install a collapsible stock of your choice on it. Just remain aware of where the battery goes and all that.
2. Funny fact, the actual weight of a "real steel" M4 is heavier than the M16A2 it replaced. Both still clock in at under 8 lbs and I suspect the airsoft versions will as well. Believe it or not, many people complain of the small standard grips of the M4/M16. The small simple grip is referred to as the "A2" grip. I'm unsure if this is actually a model designation or slang it picked up due to first being designed and issued on the M16A2 service rifle. Your best bet is to buy an aftermarket grip and swap it out. It's a little more involved for airsoft because the grip houses a motor. In "real steel" they are only held on by a philips head screw.
If you are finding that the length of pull is uncomfortably short with the crane stock, and it's fully extended, you may look finding some "buttpads" or stock spacers for the shoulder end. It might be as simple as a rubber piece that just slides over the butt of the stock and adds an inch or three between your shoulder and the firing hand.
If you haven't gotten a foregrip for rifle yet, you can attach it at the very end of the stock, practically right under the front sight, and that may open up your comfort level a bit.
If you are willing to sacrifice your crane stock for comfort, Classic Army puts out a decent line of larger weapons. Consider an M14 Match, SAR Sportmatch M41 ES, SR25, or something I'd see as a welcome addition to the sniper community here with a different kind of rifle, the
http://www.airsoftpost.com/tokyo-marui-jsdf-type-full-size-rifle-p-23961.html
Oh, and generally, this forum is only for sniper rifles and we like to keep it just for that. I'm assuming your were going to mod this M4 to be a Designated Marksman weapon of some kind...