Mod Backwards Compatibility and other Qs

[netninja]

I am seriously considering buying a Full License in the next few days, but before I do I would like to know a few things. I'm sure you've heard most of these questions many times, so thank you in advance for your patience.

1. Are modifications/hacks backwards compatible? That is to say, if a mod is listed for version "3.7.x" will it work on the latest version that I buy, 3.8.3?

Specifically I was looking at this mod:
https://vborg.vbsupport.ru/showthread.php?t=174073

If this mod works with 3.8.3, is that common? Do all 3.7.x, 3.6.x etc mods work with 3.8.x?

2. Once I buy a license can I download any older version that I want from the members section? Eg. If the latest version is 3.8.3, can I download 3.7.x if I wanted to? (This is mainly for the situation where plugins don't work with the latest version, as mentioned in Q1.)

3. I think I read something about there being a cost to upgrade to vBulletin 4? If so, will I have to pay this if version 4 comes out before my 12 months of updates runs out?

If I have to pay to upgrade to 4.x then I won't really be receiving any updates for 3.x seeing as though 3.8.3 is the "potential final version". Hence, the second part of my question: Would it be better for me to wait until 4.x comes out before buying? (Please try to give a non-biased answer )

4. Once my one-year update license expires, can I still download content from the members section? (The current version I had, as well as mods, etc.)

Thank you for your time!