Lets take a moment to review some simple facts on all of the listed AEGs.
The M4 uses a version 2 while the AK and the AUG use a version 3 mechboxes.
Both version 2 and 3 mechboxes share many similar parts so sourcing upgrades and downgrades should not be an issue.
Many have stated that a version 3 is more robust when it comes to durability when compared to the V2. I somewhat agree with that statement, however I feel that a well maintained version 2 would last just as long.
When it comes to FPS out of the box, TM is lower due to the fact that TM is Japanese made and in Japan airsoft has an FPS restriction. Springs cost only a few dollars and spring changes are not that hard to do.
Lets talk about the pros and cons about each AEG.
Easy to source external upgrades and accessories
Battery can be stored in the front or back depending on your preference
With a little work the M4 can physically upgraded and changed to perform with nearly any circumstance, be it CQB, DMR, rifle man etc.
Most M4s being sold on the market today suffer from barrel wobble. This is when the front end of your gun wobbles slightly. This Con can be fixed with a metal body and 1 piece outer barrel upgrade.
Broken Tabs, even though most plastic receivers being produced today have reinforced tabs, these tabs can still break.
Annoying disassembly, way to many parts to deal with to fully take the gun apart.
External upgrades are easy to acquire
With a little work the AK can physically and internally upgraded and changed to perform with nearly any circumstance, be it CQB, DMR, rifle man etc.
Wood kits add a very realistic feel to your AEG.
Your battery can be stored on the top of your gun or if you wish you can opt for a rear wiring and store a larger battery in the stock.
A decent assortment of external upgrades and accessories
AK disassembly can be just as annoying as M4 disassembly if not more, however this comment only reflects my opinion on the subject.
Due to its bull pup design the mechbox is seated further back in the body of the gun allowing to have a longer inner barrel.
Easy disassembly, you remove your back plate, pop out a screw and your mechbox pretty much just falls out the back of the gun. Easy access
Limited external upgrades, dont get me wrong, there are still a quiet a few addons and accessories out there to give your Aug a little more spice. There are just not as many as there would be for an armalite or an AK.
Limited battery space, from what I understand the spacing for battery storage in the aug is limited, possibly limiting the user to mini batteries only. Now I am not entirely sure on the topic but there maybe a way to house a larger battery but I cannot envision it simply going off my memory right now. If I am wrong on the subject someone please correct me.
Now although my lists are thin, I believe I have provided you with enough details to make your own decision.
"Its only a little bit on fire"