It's been forever since I've touched Quest, I'm looking forward to fiddling around with it. Is the text-based code still more or less the same or are there major changes I should be aware of? Most of the older little experiment games I have lying around are a bit wonky, but I'd love to uncover them and mess around with this nonsense again.

Quest 4 hasn't changed much so you should still be fine there. Quest 5 is a totally new system, so although you'll be able to play your old games and examples, you won't be able to edit them.

You can install both Quest 4 and 5 on the same system though.