Brandon you still have a long way to go with your recovery. It takes 2 full years for a patient to completely recover from what you have been thru. For at least the first year post rads, push the calories. Right now taking in 4000 a day isnt too much, nor is 5000. I know it sounds like an astronomical amount, but its not. If you are working out during your recovery phase you need to account for those lost calories by pushing even more daily. Adding protein will help rebuild your muscles.

The YMCA has a free program for cancer survivors thru Livestrong. There might be some experts there who could help you develop a routine to help build muscle mass. I know some forum members are really good at this type of thing, hopefully Uptown will reply soon. He works out and is very knowledgeable. I think he told me some days he takes in 6000 or 7000 calories and still cant gain weight. His situation is different (just like everyone's) he has been thru the war many times and somehow survived.

Good luck