It is not easy to get a clean finish when using a handheld tool to cut a 4-inch-diameter hole in wood. But if you only have a router, you can still get the job done. There are two main types of router you can use: a plunge router or a fixed-base router. Plunge routers allow you to set the depth of the hole and are easier to use because the base is constantly flush with the wood while you move the router up and down. Fixed-base routers, on the other hand, have a single depth adjustment and require pilot holes before you can cut into the wood.
Fix a bit to your router.
Place the 4-inch template on the wood. Mark the centre of the hole you want to cut. If you are using a fixed-base router, drill a pilot hole through the centre of your hole.
Set your router to the lowest speed setting.
Place the router on the wood so that the base is flush with the wood and the edge of your bit is on the centre of your hole or in your pilot hole. Hold the router firmly on the wood, switch it on and keep the router under control until your hole is cut to the desired depth.
Re-place your router inside your template and repeat step 4 until you have cut out all the wood inside the template.
Sand down the edges of your hole to give it a smooth finish. Use a carpenter's chisel to chip out any wood you missed with your router.
Using a router with large bits can be dangerous, especially at high speeds.