G4 is a lot more stable than it was. A couple of months ago I'd have given a firm "no", now it's a tentative yes. Triple back up your project though. Just dipping your toe in the new engine is worth it and progress you make now is good.
Here's devmar talking about his experiences porting his 3d game over and he's very positive about it although you can tell it was a slog in places: https://www.youtube.com/watch?v=Q_miIugqx3Q
There are quite a few changes, a lot can be handled trivially with shift, ctrl R tbh but others are more of a pain. Still, there's a case to be starting early.
That said, it crashes, stuff stops working = alpha. But I'd say yes, have a go and see how you get on.
Did I mention backing up your project? Back up your project.
Note, there are some auto-porters rolling out, I haven't tried them myself though. That's one thing that'll be easier closer to launch.