I understand that AI routines and commands are being reworked for version 5.0. I think AI is a very big project to address and TFS wanted to get the most fixes out in a patch as could be done in a reasonably short time. All for free if you already have Cliffs of Dover, very cheap through Steam if you just want to try it out.
Slip, there are other good reasons for downloading 4.5, including some new sub-versions of aircraft types, flyable Beaufighter, DX11 support, 64bit .exe, fix of altitude bug (very important if you fly blue), many corrections to FM and damage model, etc.
If you are flying offline, you can still have more than one version of the game installed. If you are worried about keeping them separate from each other, copy you old version to a thumb drive and run it from there.

All good advise Murph !!!

Will this version work natively with windows 10 or will it need any fixes?

Actual 4.312 run in Windows 10/64 bits without fixes other than install TF patch 4.312 - what is done automatically by STEAM enabling Betas.

What need 3rd party fix installed manually for run in Win8/10 is the latest "official" version of 12/2012. But no motive for insist in use this broken version instead TF patches.