If you are acquainted with any one of the 1974, 1978, 1979, 1981, 1983, 1989, etc., TSR editions of Dungeons & Dragons, then I can count on your knowing what I'm talking about when I'm talking "AC4, HD3+2" and so on.
Okay, but what's the use if the stats don't represent the power level of the game system? Then they might as well not exist.
There's no such thing as a "Hit Die" in 4E, and no such thing as most of the 4E jargon in old D&D. Is there a "Weaponsoul Dance" or "Chains of Sorrow" rule even in 3E?
Yep, however you don't need to know anything about it except what's included in the stat block.
If you are able to read a 4e monster stat block, you're good to go.
What actually happened was that TSR sold both sets, PLUS the modules and magazines and so on that were easily usable with both sets.
Okay. But in my completely anecdotal experience people were not mixing anything. In our area it was like this:
Group 1 played AD&D with person A as the DM, and the players B, C, and D.
Group 2 played D&D with person B as the DM, and the players A, C, and D.
Group 3 played TDE with person C as the DM, and the players A, B, and D.
Group 4 played Midgard with person D as the DM, and the players A, B, and C.
Now, those players who had the cash available would buy stuff from AD&D, D&D, TDE, and Midgard.
Those who didn't, typically didn't buy anything but were supplied with photocopies.
Sometimes players would buy stuff for systems that seemed to be played by many people, so they would have a clue and be able to join a group when a space became available with a minimum of guidance.
DMs were in short supply, so you typically had to adopt their favorite system if you wanted to get to play at all.
There was also a very high percentage of completely homebrewed rpg systems. Official adventure modules were an exception rather than the rule. Almost everyone wrote their own adventures.
As soon as I had a job, I started buying pretty much every rpg system that looked remotely interesting, becoming a collector.
I also started buying adventure modules - but only for inspiration, not to actually play them. I also thought that D&D adventures sucked big time compared to the modules that were released for other systems (back then I particularly liked the MERP modules).
Anyway, I think I just lost track of my train of thoughts...
I guess what I wanted to say is that compatibility was a non-issue back then. Noone expected nor wanted adventure modules to be compatible with anything.
And I still fail to see, what the hell should be incompatible between two modules written for D&D 4E?!