My four year old has a normal temperature in the morning and by the evening its 99-99.5 in the mouth, or 100 rectally for a few days now. why is this? currently he has has no other symptoms and still just the elevated temp. eats, plays nothing hurts?

There is a variation in temperature during the day and thermometers may not always be accurate. A screening examination to ensure that there is no infection may be helpful. UTI and a mild URI, COVID 19 should be considered. Watch for signs of vomiting, keep your son well hydrated.

I don't consider this temperature range a problem. If the kid appears healthy, feeds well, plays normally and has no symptoms I would put away the thermometer. The more you focus on fever or illness in an otherwise healthy kid the more you make them think something is wrong with them.