How to play steam games without updating
The Beta is a good warning that a new update is due, so unless you want to help find and fix any bugs for mods and/or the game.
Waiting until the PC release and checking none of your mods have outstanding problems, makes sense.
If we want a mod, only available on the Workshop, We subscribe only long enough to download and archive (,7z, or .zip) the 2 files all Workhop mods must have (and which is now .ba2 in Fallout 4).
Mods here on Nexus and otther sites can come as loose files and folders or packed in a .bsa/.ba2 file.
Generally the less experienced you are the longer you wait.
Other more experienced modders, will test the game for changes.
Indeed you can't turn off auto updates for Steam at all, the best you can do is limit them to one hour per day (Downloads Tab in Steam's Settings).
If possible I recommend getting games you want to mod from GOG, their DRM free games are ideal for modding.
We cant' get around it with this system with this game, UNLESS the developer puts up a past branch system one can use. For now, I will live with console games for simple what not when I decide and be keeping a eye out for simple texture swaps. In a Bethesda Game Studios post, I saw on Steam Thanks and Updates for Fallout 4 | They confirm that updates will be released first as a Beta, then PC and last Console.
European Truck Simulator developers do this, one can go to the 'beta' branch on Steam for that game and force Steam to keep you on whatever older version of the game you desire (within limitations going back in time ) So it is something a developer could do... Pretty much the same as happened with prior Fallout games. that they would be releasing many smaller patches rather than a few big ones.
GOG Galaxy allows you to set Auto Updates for each game and they provide manual installers as well, it's up to you which you use.
Falout 4 doesn't have that option and Steam doesn't allow you to turn off Auto Updates for just one game.
Sadly I doubt Bethesda would have the vision to do this. This should make it easier for modders and Bethesda to find the cause of bugs in each patch.