If I'm not mistaken, chapter 4 full does not contain anything new, only a reworking of chapter 2. I ask because when I try to load some of my advanced games I get the typical error message, it is necessary that I play it all again so that in future updates I don't did i get the error?
The thing is.... Ocean is reworking the game while he develop its further. So the last update always contains some reworked (or better said A LOT) content, this time chapter 2, next time the rest of chapter 2, and we will see if he wants to rework soemthing out of chapter 3 too, but i dont think so.
So to use an old save is either way not useful. Cause the changes are rather big. I know its annoying to start over again, but nobody is pressuring anyone to do so... instead you could just wait until the rework is done... whenever that is. Or live with it, and start again...
But i think starting all over is not that big of an issue... for now setting up 2 different playthroughs is enough so get nearly everything without missing anything meaningful.
I enjoy the rework overall very much and i think Ocean had a pretty good idea why he is doing so. Some prefer the previous versions, but the longer the game goes, the more sense makes the rework, i still have the old version, and must say, how the MC acts there in the first chapters doesnt fit him anymore later on... with the rework and added characters it all is a lot better in the flow.
But there are som continuity errors Ocean needs to get rid off, but nothing major...
there is no "new" content after ch 4.5... BUT the rework is not just a "rework" in improving the renders or such... no there are a lot new scenes and even characters added. So this is new content, but before 4.5.