I have had this tube of MX4 since 2013, it's served me well, countless rebuilds of my computers, CPUs and GPUs alike, home servers, gaming computers, laptops, games consoles, pi4, I used this on everything. You served me well.
8th June 2013 to 4th November 2023.
I didn't notice any breakdown of the paste. It typically dries out over time once applied, this was still the same consistency throughout its life (in the tube)
This has been my experience with MX-4 as well. It's really really really stable. I've never had to re-apply MX-4 due to it drying out, either. (It's supposed to last at least 8 years in use.)
The same tube has been sitting in my drawer and every time I've needed it, every few years, it's been good.
I have decade old tubes. Maybe some bits around the entrance can dry out if you didn't clean them after use but it doesn't seem to be an issue for mine as far as the inside stuff goes (of course I put the cap back on as well).