To answer your question, 50k training is essentially the same as marathon training, with extended long runs, and more emphasis on nutrition
It's important to not forget track workouts, tempo runs, strength training, etc. The farther the distance you're training for, the less those matter (with the exception of strength training - that always matters)
Running sub-4 in the 50k really depends on the course. Assuming elevation is minimal and the race isn't technical enough to alter your stride, then your marathon time should be in the 2:55-3:15 range; 2:55 if long distances aren't your specialty, or 3:15 if you find you're more competitive in long distances as opposed to 5ks or 10ks or you find that you consistently negative split your longer races
With 50k racing, nutrition is even more important than it is for a marathon. They say the best ultrarunners are the ones with the strongest stomachs, and that holds true time and time again. Find something that gives fast-absorbing energy and is easy to digest
Long runs - With 50ks, you have a two options. Slow and long (say, 8:15 pace for 28 miles), or shorter and slightly faster (7:30 pace for 21 miles). Either will get the job done, but the former will give you more confidence. If you could, try to incorporate both into your training!