How do you change a 3-prong stove to a 4-prong?
You can simply take the back panel off of the stove. The three prong cord will unscrew from the posts. Purchase a new 4 prong cord and connect it the same way the old cord was attached. You will have an extra wire. 4 prongs equals 4 wires. This extra wire is an additional ground that attaches to the actual stove. It is usually green. You will see a screw on the back of the stove near the 3 posts that is screwed into the frame of the range. Unscrew that that place green wire there and screw back in.
Most ranges are sold without the cords and three or four pronged cords can be attached interchangably based on the type of outlet in the individuals home. Appliance stores should have both types of cords available for purchase. If they are helpful people they should be able to explain the installation as well. Sometimes that's best if they can visually show you. Better safe than sorry when dealing with electricity.
Four wires = two hots (red & black), one neutral (white) and one to the stove frame (green). Just connect red black and white the same way they were before and add green to the frame of the stove. There should be a screw in the frame near the electric post often the screw is colored green.