The issue is that there are so many solid M4's out there, that it's actually easier to compile a list of manufacturers/product lines to not buy. Also, you don't mention your budget.
So I will tell you a few of brands that I would personally avoid:
- VFC: look great externally but are made of cheese internally.
- Ares: Funky electronics (that will do 3rd burst) but are made of cheese internally.
- KWA: Apparently are reasonable guns, but proprietary parts can be a ball ache for upgrades.
- Krytac: See KWA
- SRC: Nasty
- Specna Edge**: Looks ok externally (and loads of rail/length choices), but are wank internally. Once re-shimmed, greased and a fair few parts switched out, will work ok until the gearbox shell breaks because...you guessed it... it's made of cheese.
Again, this is my personal avoid list - I'm sure the others will disagree.
**Note, my criticism of Specna is leveled only at the Edge Series. The 'Core' and 'One' series are supposed to be reasonably decent as they have (according to rumour) different OEMs.