mostly just the look, and the length, any m4 can be changed from a cqb type to an sr type and anything in between, baisicly just swap out the inner and oiter barrels and then put what ever flavor of handguard/rail system u like, i personnaly use a stag15 with a daniel defense mfr12inch handguard/rail system, it is baisicly an m4 with a crane stock and a long barrel looks like this, mine has a small barrel extension and a madbull amplifier to hide a longer inner barrel.
all the airsoft m4's will have a version2 gearbox and a 363mm inner barel some will have a longer (sr types)or shorter (cqbr types) inner barel tho some with the longer outter barel will still have the short inner.