Let's get right to it, stop these four exercises. They all stress the discs in your lower back that will likely make you stiff and sore later. Yes, they feel good at the time, but it comes at a cost!
1) Toe Touches - bending forward to touch your toes stresses the disc and weakens the back part of the disc which can lead to disc problems. Don't worry about occasionally bending over, just don't try to force a stretch in this position. The same applies when you sit and force a stretch to touch your toes. Here's a better alternative.
2) Pulling your Knees to Your Chest - same as above. Here's a better alternative.
3) Spinal Twists - Lying on your back and twisting so your knees touch the floor. Your lower back is designed to be a stable support for your hips, legs, shoulders, and arms. This movement stresses the discs and can irritate the joints between the vertebrae in your lower back. Here's a better alternative.
4) Sit-Ups - stresses the discs, stresses the neck, can separate the mid-line of your abdominal ( core ) muscles. Here's a better alternative.