If you purchase the base model of the Tokyo Marui M4A1 MWS GBBR, you will most likely change the outlook immediately and aside from the foregrip and the receiver, the outer barrel will most likely need replacing. If you're the type to switch things around every so often, changing the outer barrel length from the standard 12.5 to maybe a 4 inch barrel, then the First Factory Real 8 Way Outer Barrel is the perfect solution.
This particular outer barrel comes in 4 parts that give you a variety of lengths:
4 inch
5.5 inch
6 inch
7.5 inch
9 inch
10.5 inch
11 inch
and finally, 12.5 inches.
Standard M4 to PDW, there's a lot you can do with this kit!