Seeing as we seem to have followed a pro /con format earlier in the thread...
Reasons for getting an M4 as your first gun. Good choice from budget to obscenely expensive with AEG, HPA and gas rifles available. There's a huge range of accessories available for the exterior of the rifle, sights, stocks, fancy receivers, grips, rail systems... In fact if you're inclined you can build a gun completely from scratch (although this way madness and poverty lies).
Reasons not to get an M4. Everybody, their friends, their relations, colleagues and that bloke down the pub no one talks to, has got a bloody M4. Which means whatever combination of parts and accessories you come up with means that someone, somewhere has done it before. It's the airsoft equivalent of two women turning up to a wedding wearing the same dress.
At some point you're going to want to establish a bit of a 'look' (face it, most of us are grown men running around in the woods with toy guns that actually pew pew, so dressing up as well...it's a small step) and unless you like being a clone that's always going to be tricky if you've got the same kit as everyone else. But to be honest, if you reach that stage there's no hope for you. The Amazon delivery driver will have worn a groove to your house and will be on first name terms with most of your family and neighbours. A string of credit card companies and the bank will be angrily demanding more than the minimum payment. Any significant other will have either left or developed a resigned, far away look and a good line in eye rolling as she prepares beans on toast for the fourth time that week because it's all you can afford...
Hence the AR-57. Take a perfectly good rifle and make it worse and ugly... but at least it's a bit different. Personally I'd rather suck farts out of dead seagulls. Although for some obscure reason you can actually buy an airsoft version.