In the way of burst, for gas blowback, I dont know of any M4s that have that function. The new .308 by KWA has 3rd, but that obviously isn't an M4.
In the way of AEGs, very few high end guns offer this. But those that have the option out of the box include:
-G&G newer generation guns, Predator, SRS/L/XL. These have the G&G ETU (Electronic Trigger Unit) that has a trigger board instead of contacts, resulting in a short trigger pull and cycle completion. The unit is programmable by pulling and holding the trigger in semi for 5 seconds to switch between Safe-Semi-Full auto and Safe-Semi-3rd Burst.
Really the only downside is the bulkiness of the wiring and mosfet unit itself in the buffer tube.
-Ares guns with the E. F. C. S. (Electronic Fire Control System) This includes the Ares OEM EMG guns (Hellbreaker, etc), Ares "Honey Badger"/Amoeba line, and possibly others.
The EFCS has active braking which offers better motor control for the system, at the cost of often times more heat build up. But for this system to be programmed, you have to buy the separate programming unit which is ~$60.
-Umbrella Armories gun with either BTC Spectre or Chimera, or the Gate Titan. These guns are completely rebuilt Krytac guns, at the price of ~$1000-1200. These are pretty much one stop shop, no need for upgrades, pick your performance, and buy once, cry once. Programming for these can be done either with bluetooth, trigger pulls or a programming card/USB connection.
Past that, there are a number of aftermarket burst mosfets that can be added to any AEG for the most part. They consist of:
-Plug and play, which don't protect anything, just offer burst or whatever functionalities it is advertised for. These are based on system cycle timing as opposed to cutoff sensors, so are not as accurate or as consistent as those which have sensors in the cutoff area. Some offer the ability to be hard wired in, meaning adjusting the wiring to actually offer protection to the trigger contacts as well as all the other, and possibly more features than just plug and play.
I don't recommend most of these, but they are on the cheaper side from $20-50. Of the ones out there, I only recommend the Gate Merf3.2.
-Drop in units. Such as Jefftron Processor, BTC Spectre, Gate Titan, and some others not as common. These units offer a plethora of features including burst, active braking, battery protection and more. They replace the trigger contacts with a trigger board on which all the electronics are contained on so the wiring is clear of any large chunks of electronics and guide cables, allowing for more battery space.
I highly recommend the BTC Spectre MKII if you can get one, but sell out fast when they are in stock anywhere and get sold on ebay and elsewhere for $180-200+. The Gate Titan offers similar features, but is more accessible as it is produced by a well established company where as BTC is a smaller operation, so product batches come far and few, with distributors and retailers having first dibs.
There are other mosfets out there, but I'll leave this for now.