As I see it, it is all about preventing overtraining, so giving your body enough rest to recover. Typically, a high intensity training "damages" muscles and forces your body to repair the damage and go a little bit further to prevent a similar incident next time. Typical repair time is 48 hours, but with aging this can become longer. If trainings are stacked too closely, there is no time to repair and you are breaking down your body. If you wait to long to use the muscles again, you will be degrading as well. A low intensity training will force bloodflow and remove any damaged tissue, but won't inflict harm. A 3 trainings a week allows for such 48 hour repair times. A 4 times a week doesn't, so you probably are better off doing some low intenity training, just to keep the bloodflow going and activate the muscles but not damage them.
Typically, most trained athletes are taught to "listen to their body": when muscles still feel stiff, wobbly or tired after a warmup, don't do high intensity trainings as it is a sign of muscles still repairing themselves.
