I know nothing about M4's so take the advice of Flazz and Dee above, do some research yourself via reviews on other forums too, as there will be more experience around the world with brands we see less of here and may open up some extra options for you.
Most importantly of all though, please bear in mind that the law changed a while back preventing importing any realistic looking firearms which includes airsoft weapons, without a permit issued by the police. This leaves you with 2 options, 1: buy from a reputable stockist in NZ or 2: Play a few games at a club (I recommend TAG!), if you like what you're doing, join the club (again, I recommend TAG!) and then with the assistance of the armourer you will be able to obtain an import permit.
Option 2 is, in my opinion, the best way forward. Airsoft can get expensive, and there are so many different ways to play, and different weapons to suit different styles. Everyone at TAG is super friendly and more than willing to talk about themselves, and their weapons, sometimes you can't shut them up. Most will let you try their guns too, giving you a really good idea where you'd like to spend your money. Obviously you've already narrowed the field somewhat with wanting an M4, but as I'm sure you're discovering, the M4 field is pretty wide.
TL:DR - To import an airsoft weapon you need a permit. Come out to TAG for a few games before doing anything else and decide from personal experience what you like and don't like.
