I've never flown exactly this combination before and my last trip to ZA is quite some years back, but Airlink (4Z) seems to have a Baggage Interline Agreement in place with Delta. So I'd expect them being able to check your luggage all the way through.

See also here:
https://www.flyertalk.com/forum/africa/1009900-jnb-johannesburg-airport-connections-facilities-layovers-etc-77.html