M4 parts should be mostly compatible
My CQB primary is a ~2008 UTG Sport model 4, perhaps sharing some notes on the various little odd things I learned while building it and others would be valuable here - only the upper and lower receiver on this gun are original at this point, everything else has broken and been replaced over the past however may years. I've got over 1k in it in parts last I added it all up and it's the sweetest little gun to shoot... Here's some things I've encountered which I wouldn't have considered beforehand.
The disk on the back of the selector switch (the part that interfaces with the selector plate) was a smaller diameter on the UTG than others which caused the gun to be really finicky when switching from semi to AUTO in some configurations. I also learned that some are thicker than others as well, chased that issue around and around until finally finding a lonex switch set that fit the receiver and was the right size to interface with my gearbox of choice (also Lonex at that point, go figure).
The trigger pin in the original gun was a hair thicker than in several aftermarket gearboxes I've had in it over the years, had to either bore/ream the gearbox or run a thinner pin. Did both at different points, currently bored out which means the gearbox will be loose in any other receiver / cannot leave this gun until it breaks.
I had issues with a few gearboxes where the back / where the bolt from the stock goes into the gearbox is shaped differently between OEMs, some shells sit nicely in the lower and some tilt back when the stock is tightened. I eventually drilled the back of the receiver out and tapped threads for a large grub screw with a hole through the middle which allows me to adjust the angle of the gearbox as needed. This is a great solution, but holy heck is it important to get that hole tapped straight.
Triggers are always a issue if you don't use a stock curved style, and sometimes even if you do - I've filed a good 5mm off the front of the cutout in this gun to make room for a dozen or so different bang-switches. Trigger guards, I like the 'enhhanced' version with a slight curve to the bottom as it keeps my hands from slipping forward when working corners - turns out there's a number of different widths there as well and that's a spec that is extremely rare to find listed.
Magazine releases are tricky, some are interchangeable and some just aren't due to being slightly to thick, to long, etc. Got lucky and a Lonex one again fit this receiver's cutout perfectly, but even there the ramp was to steep and it made loading mags feel really chunky. Fixed it by filing it way down, gave it a much more gradual angle.
Finding the right nozzle length can be a pain when things are mismatched, past solution was to get a polymer unit and use a special bevel-bit (works like a traditional pencil sharpener) for my dremel to custom fit it. Fortunately the Maxx chamber I'm using now hasn't given me any real trouble, pairs really well with the receiver and gearbox.
Outer barrels, need to keep an eye on the backspacing, the rim or the 'lip', and hop chamber cutout. Some 'lips' are thinner than others so when the nut is fully tight, the barrel is still loose - shims on the front of the barrel will fix this. Sometimes the backspacing is wrong and the outer barrel actually extends into the upper receiver a bit; there are special shims to fix this but they are few and far between these days if you're looking. I'll post a pic if anyone is interested, pretty obscure. Hop chamber cutouts are a mess - some are deep, some shallow, some wide, some tight... and naturally differnt hop chambers are longer, shorter, wider, etc etc as well. Finding a good fit can drive you mad lol. Also (and I assume you know this, but just to cover it) airsoft upper receiver threads for the barrel nut are not all made equal- some are actually mil spec, some are 'airsoft spec' which is smaller iirc, and some are just sloppy cast threads that it's hard to get anything to grab on. As a result, odds of one handguard fitting another upper is 50/50 at best.
Rail height over bore is also highly variable between OEMs, meaning the top rail on the receiver isn't always going to be level with that on your handguard if you want the flat top look while mixing and matching. Some are good, some are close, and some are 1mm higher (or lower) and just look terrible.
Motor grips are occasionally different. This is slightly less common to see now that many guns come from the same factory, but the cutout in the receiver that the grip goes into can be longer, shorter, taller, etc. Different gearboxes can also be a bit longer or shorter here too, resulting in a ugly gap around your grip or the grip not quite fitting - or even worse, appearing to fit but the motor angle is all messed up.
Charging handles are usually somewhat proprietary, have had very little luck swapping between OEMs on any of my guns.
Dust covers, there's 3-4 different latch styles and some variation on tab length as well. Have to pay really close attention when selecting one for a different upper, odd's are high it wont fit. Should also mention, the dust cover pin can be different diameters and there's a couple different ways they are mounted too - most use a c clip, but the size of that clip is also different.
Takdown pins! omfg, takedown pins. Every OEM seems to use a little different style, length, etc. If you have to replace the original ones for whatever reason I've had the best luck with locking pins as they are by nature somewhat more forgiving if the length isn't perfect - but even then I usually have to sand them down for mixed builds. King Arms pins aren't always the right length for King Arms receivers, etc etc.
So yeah. All M4's are basically the same, but the size of the asterisk next to that statement is freaking yuge when you really dig into it and start trying to fit parts together. You're going to find this a very frustrating process at times, but if you push through it, if you remain committed to the build... In the end, it's worth it. Might not look like much to anyone else, might just seem like any other black m4, but to you - it'll shine.
Here's the gun I referenced btw. Currently 16:1 1j DSG, 21t, ~48rps on a smol buffer tube stick
And this is how it was ~15 years ago, 18:1 1j SSG, 28tpa, ~12rps on a 8.4v NiMH boat anchor
Not the same gun, but same model and year - also mine 
