It's likely still makeable if you arrive on time. 4:50 is slightly past peak arrival time so lines should be down at immigration. Reclaiming bags and customs are rarely, if ever, a problem at NRT. Still, I would try to get from the plane to immigration as quickly as possible.

Unless you are travelling in paid F or have some super-secret status, I wouldn't expect much out of Delta in the event of late arrival.

If the arrival time of the DL flight was changed but not the departure time, it's quite possible that it was done solely to make their on-time stats look better. You'll still probably arrive at the same time as before.

Welcome to FT, flyer12345678--what's next?--9!