AFAIK, I need to update the launcher from Steam, otherwise the launcher itself won't connect to Frontier's servers. I'm pretty sure it's all or nothing - if I want the new launcher, I need to get whatever else is in the update. Normally this is a good thing, because Steam is very fast. Compare / contrast to MSFS, where only the launcher is updated via Steam, and then the launcher itself downloads the rest of the FS updates from Microsoft. ESO also is a "the launcher does the updating" title, but Elite isn't, not on Steam anyway (again, normally a good thing).
The other problem is that even if disabling updates from Steam does work because Frontier continues to allow connections from the old launcher, someday there may be an update to 3.8 that I'll need, which means I'm just postponing the inevitable if 4.0 is a mandatory update.
It just feels like installing 4.0 is going to a mandatory install for Steam users. Now maybe I can manually (re)move it afterwards (I'd probably push it onto an external drive) if I need to free up space. IIRC 4.0 is an entirely separate install from 3.8, with its own folder and config files, so this should be an option, yes?