I believe the idea behind G4 Sea Lion is to ensure you prepare adequate defenses against Russia. If Japan violates the NAP, all Germany has to do is hold Baltic States, Bessarabia and E. Poland (to prevent Russia from getting NOs for Europe territories).
Assuming you own the Atlantic, putting a submarine in SZ 125 will stop Russia from collecting more than the 1 NO for taking Norway (if you let them take it.) Eventually, Russia will have to send something back against Japan and then it’s a matter of getting a strength so Germany and Japan weaken and Italy takes Moscow. (Hey, you gotta figure Germany has Karelia and Stalingrad so Italy is the one that needs the complex the most, right?)
Just my opinion, never actually went the G4 route, but it does seem to be more secure and does not commit you as readily. (You can go 50/50 tranny/military two rounds instead of 100% tranny one round.)