how to manually enable debug mode on galaxy s4 with black screen?
I would comment this but i don't have enough rep yet.
Off the top of my head i'd say your best hope is to watch a youtube video of someone doing it and try to replicate the touches, for a sense of if your hitting something you can turn vibrate on (using the volume controls). Other than that you could always claim warranty or try to get a new screen. 
Hope i helped