The MGC M4A1 is a very good modelgun, very detailed and realistic. It is also firing quite reliable. It has two drawbacks anyway:
1. it is plastic (although looking and feeling good it is still plastic)
2. the HW plastic material is very brittle and can break (extensive use will wear the carrier bolt finally and if you drop it, be sure something will break).
But if you want a M4A1 modelgun, the MGC is the way to go.
What do you mean with "parkerised" finish?
Early and very late M4A1 models were sold in natural HW colour. It is a rather shiny metallic finish that is not too realistic. Anyway it will get dull (and looking more realistic) when oiled. Modelguns-worldwide call it "military finish" which is odd in my opinion.
Other models were flat black. I like this finish and think it looks quite realistic. I have both versions and really like both of them.
Black finish:
"Metal/natural HW" finish (note this is a M725 "M16A2 Carbine", not a true M4, but you can see the finish here). I do not have a pic of my M4A1 in this finish: