http://www.actionhobbys.co.uk/Adjustable-Pressure-Operated-Laser-With-Weaver-Mounts_A5NZ8.aspx
Hey, both of these guns are made by JG, a good company so they will have similar internals and fire pretty much the same FPS. I think quality wise they are both great guns, and they should take similar mags so that shouldn't be a problem.
Now, about the mounting the items, this will be much easier on the HK416, and they will look much better mounted on that. The M4A1 is not made to have those mounted on.
So, for your case I would definitly reccomend the HK416.
Hope this helped
*and like the guy below me said, the two guns have similar internals, but the HK416 would be much better for mounting the items you want to.
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