Its all going to come down to how well you recover, how intense your training is and how much food you are getting. Over training is hard to do, especially if you are getting a large surplus of food and still able to rest. I would push as hard as you can, but constantly watch yourself and see how you respond. Take breaks when needed. I can tell when I need a day off, usually after a very heavy several days of training. My CNS takes a hit. I feel like I'm coming down with a cold. Everything aches. I feel weaker and my appetite goes down. Still, if I take a day off and bounce back the next moring, feeling strong and ready to kill it, this wasn't over training IMO. I've been over trained after a contest and it takes weeks or months to recover from. If you can take a good rest day and feel back to normal the following morning, you aren't over trained. 

A five day a wk training program is pretty normal and for most people, they respond well to it. Some guys do need more though. I've met guys that swear by 3x wk training. I'm talking big guys. One guy who comes to mind is kali muscle. He said that he made his best gains while training 3x wk. For him, it worked. For me, I need more. Its just what I need. Keep in mind that at 45 yrs old, you aren't going to recover as fast as you did 20 yrs ago. If you are killing it in the gym 4x wk and feel yourself brushing up against your limits, then so be it. 4 days it is. 

If you normally train 4x wk and want to add in a little additional work to bring up weaker links, I would suggest you keep the 5th day to higher volume, non joint taxing movements, focusing more on bringing fresh blood into the muscle, keeping it full and pushing nutrients in. Believe it or not, this can actually help speed up recovery. Just make sure you still get plenty of rest and plenty of food