WildHobgoblin: The problem and how to fix it (if anybody happens to run into the same issue)

So, I fiddled around with this a lot, and it seems at least in this particular case, the problem is not anything you do or don't do in Path of Destiny or Ripples, it's actually the Alvin quest that is screwed up. There's a possibility that there are other ways you can get stuck in chapter IV, but I'd venture a guess that this is what usually happens.

There's a non-optional (!) conversation you need to have with Alvin that serves as a trigger for progressing in other quests. If you progress Ripples/PoD/Alvin before you've had this conversation, the game will not tell you that this is what you've missed and the game will progress as if everything was in order - until you're completely stuck.
There seems to be no way to get this conversation to trigger in the game after you've progressed past a certain point, no matter who you talk to etc, so the situation is basically irredeemable. But - you can mess around with the files ;)

How to fix it:
So, I know nothing about computers, so there are probably better and smarter ways to fix this, but this one worked for me, and has the benefit of you not having to replay large chunks of the game.
Obviously, making a backup of your save file before meddling with it is probably a good idea ;)

1.) Download the Witcher Save Editor
Download Qst Viewer (You don't really need this tool, but I found it very convenient)
Download KGFF Editor

2.) Open the save in question in the Witcher Save Editor (the readme included with the files explains how to run it)
Click Action->Unpack Save and unpack to the folder of your choice

3.) (Only needed to confirm the problem is indeed the missing Alvin conversation)
Start Qst viewer and open q4003_alvin.qst
Click your way to "on a bank" (pic 1), there are three items listed (Witcher 1,2,3) - exactly one of those needs to be "completed". If none of them are, then yep, that is indeed the problem. You may wanna check the dialogue options displayed for each of the choices, since you'll have to pick one of those in the next step.

4.) Now, run KGFF Editor and open q4003_alvin.qst . Then click Tools->Search by text "bank" and click your way down until you arrive at pic 2.
From there, scroll down slowly. You'll see some dialogue about the fisher king - scroll right past that, we don't need to tinker with that. Scroll a little bit further down, until you arrive at pic 3.
Now this is where the magic happens. You'll find the three dialogue options further down (each will appear twice), "I told Alvin that no one..." "I told Alvin that becoming..." and "I told Alvin he should..."

5.) Scroll down to the dialogue option of your choice. Above (!) that dialogue option
change value of "Completed" to 1,
change "Marked as True" to 1,
change "CurrPhase" to 0.
(should then look like pic 4).

6.) Scroll down again, underneath (!) the dialogue option of your choice
change "QISDFTP" to 1
change "QuestBegan" to 1
(should then look like pic 5).
In the example in pic 5 I chose the third dialogue option, hence why it states "Witchers 3" there.
Now, save the repaired qst file (don't change the name, obviously).

=> And bam, your save is fixed. In qst-viewer, in q4003 the dialogue option of your choice should now be marked as
"completed". Repack the save with the save editor (make sure you've replaced the broken q4003 file with the fixed one).
Start the game, load your repaired save and everything should be fine :)