What are recommended exercises for someone with a herniated disc?

In principle, low or non-impact "horizontal" exercises (swimming, yoga, etc) are safer for your lower back. You don't want high impact "vertical" exercises (running, racket games, etc) to stress your worn out L4-5 disc further ! Heavy weight lifting stresses your back too if done upright "vertically" better to do them lying down "horizontally" or without bending your back.