Some sort of assault rifle is generally advisable as a starting gun for the same reasons they're used by the majority of soldiers in the real world: they're a jack of all trades weapon that does everything sort of well but doesn't particularly excel at anything. However, I don't think getting an m4 as a starter gun is necessary and AKs, SCARs, g36s, L85s etc. are plentiful and just as good in airsoft. I'm always an advocate of getting the gun you like aesthetically; find one you like and then find a manufacturer who makes it well; I'm a big fan of Cyma, as they do a large selection of guns and they're generally fantastic out of the box, especially for the price. I've had an AK, mp5 and 2 m14s from Cyma and every single one of them have been great guns.
On that though, also think about how you like to play, so I'd generally advise renting for 3 games to get your UKARA before buying. Renting will allow you to experiment with different ways of playing and then make a more informed decision when you purchase. Want to play high speed and aggressive? Maybe an SMG like an mp5 or p90 is for you. Unlike in real-steel shooting, barrel length and calibre don't really matter; a stubby mp5 will perform just as well as an m4 or AK.