It's a dangerous game trying to max out the fps for a certain limit. You run the risk of going over and being unable to use it.
I'd aim for about 350 instead. Then you can use barrel bore to up the fps. Just get an M100 and tune everything else to perfection and it shouldn't make a difference.
Use .25 BBs as well, they offer considerably greater accuracy and range. I find that guns shoot the best when they're firing .25s at between 310 and 330fps.
Your biggest issue is that Version 2 gearboxes, which are what M4s have inside them, were designed by Satan, with the idea in mind that they ought to be the most gigantic bastard to dismantle and reassemble out of every other breakdownable object in existence. Which means that taking one apart for the sake of a spring seems a very foolish thing to do.
Theoretically, an M110 ought to get you closest, then a tight bore barrel should close the gap. But you'll probably end up shooting hot. You'll likely need to cut a coil or two off the spring or something in order to get it right. It'll take a lot of repetitive, system of elimination-y-ness, which will be covered with a blanket of malevolent range and frustration until you get it right.
My advise: Leave it as it is, get a 6.01mm tight bore, maybe get a silencer so you can get a longer inner barrel, meaning the air pressure builds up longer to give you greater fps. Get something like a 509mm barrel with a bore of 6.01 and a giant silencer to hide it. Upgrade the hop, get a scope and tune it for use with .25s.
Job done.
Then, if you ever want a standard M4 back, all you have to do is break it open, swap the barrel back to the standard one, which is a 1 minute job at the most and you're ready to go.
