السلام عليكم ورحمة الله وبركاته شيخنا أحسن الله إليكم If someone has a chronic illness causing fatigue most of the time, so he can only do beneficial things for 3-4 hours a day and rests the rest of the day, doing things that may be a waste of time if he were healthy, is he sinful for this?

No, you are not sinful, and what you were regular upon when you were healthy is written for you in full for as long as the illness prevents you, as the Prophet ﷺ said that when a servant falls sick or travels there is written for him the like of what he used to do when healthy and resident (al-Bukhari). The rest is part of your treatment, so do not measure your day against the day of a healthy man. Also if reading does not affect your health after consulting with your doctor, you can read in sha'a Allah as a form of relaxation as well.