AK and G36 are version III cylinder heads and M4 is version II.
Although my cylinder head in my G36C broke and when I went to replace it with a version III, it was too tight for the nozzle (it wouldn't allow for free movement of the air nozzle once the gearbox was back together). I replaced the head with a version II and it's working quite well.
The brass piece that holds the nozzle will typically be longer on a version III.
The air nozzles are also different, not only for each version, but also for each product.