With a healthy fermentation, 4 weeks is more than enough time and it would definitely be ready to keg. "Typically" fermentation completes in under a week, but the additional conditioning time in the fermenter allows the yeast to clean up any off flavors in your beer. I usually keg after 3 weeks. To be sure, you should be taking hydrometer readings to see if you're at your final gravity.
Also, you should release the pressure in your corny key and open it up for cleaning. Warm water and PBW will remove any crud if you bought a used key, and will remove any oils on the metal if you bought a new keg. Soak it for 30 minutes upside down, then flip it upright and drop the lid and poppets into the PBW and let it soak an additional 30 minutes. Rinse well, and then fill with sanitizer. If you're not doing a pressure transfer, then only fill the keg with a gallon of sanitizer solution. Close it up and shake it around so all the internals come in contact with the solution. Then open it back up and pour out the liquid and most of the foam. It's okay if some foam remains in the keg.
Finally ready to keg your beer. If you can, please don't pour the beer into your keg. This will introduce a lot of oxygen into your beer and it will go stale very quickly. A better method is to use a siphon tube and fill the keg from the bottom up. There are tools called racking canes and auto siphons that will make the transfer much easier. I would consider purchasing one if you don't already have one.
There are advanced methods for doing pressure transfers to the keg, but that involves additional equipment and techniques. I'm assuming you'll explore those methods when you're ready,