30 secs is not much more than a rest/pause. I can see why you're failing at 4 on your second set. Try going closer to 2 minutes. Bench is a taxing exercise on the ATP. Refer to this chart and you'll see you're only 50% recovered after 30 secs.
personally i dont start another set until i feel i can put another 100% into it again. il usually wait sometimes up to 3-5 minutes before doing another set on DB incline bench depending on the weight. and the last set il usually do a drop set like 6x120 drop to 12x50 while im resting il keep flexing the pecs to keep the blood in them so i dont loose my pump... my .02, works great for me!
If you are rest-pausing sorta like DC, then you shoudl see a 50% reduction in the number of reps between each segment. The reccommendation is 15 breaths between segments for dc. Endurance wont help this=-) And this is a perfect reason i ONLY do 1 work set.