I have two sets of low caps that are for M4s.
The first and oldest have done me sterling service with no issues or problems other than one splitting open when I was a bit aggressive with a speed loader. These are ASG Armalite Low Caps. Cost me about £40 for 10. They look like metal mags but are plastic. Had them a few years and still use them. In fact these remain my go to Mags.
The new ones are the Nuprol 30/125 changeable N Mags. Made to look like magpul magazines. Only used them a couple of times and they work well. I brought them more to stop me carrying loads of mags to games (lows and mid caps). They are more expensive than the ASG ones about £12 each.
Hope that helps.
CBH