M.P.
I suggest that his temperature is well within a normal range. http://firstaid.webmd.com/body-temperature
Here is a web site explaining about body temperature. I suggest you're using oral normal temps for comparison and you're taking his temp rectally.
The doctor has already told you that playing a video game can raise body temperature because the player is excited. I suggest that you accept that for now. I do know that my grandson's face gets flushed when he's playing on the computer. Seems natural to me. Excitement raises blood pressure.
My grandson, who is 7, has always felt warm to the touch off and on. We just consider it normal and haven't taken his temperature. He may have low grade temps but because he's acting fine we aren't concerned. You've taken him to the doctor and have ruled out illness. Try to relax.
I strongly suggest that your extreme concern is contributing to the problem if it's even a problem. My doctor has told me that low grade temperatures are normal and not necessarily a sign of a fever. Could you try letting go of your concern for a few weeks and see what happens?
At age 4, it's not a good idea to continue taking his temperature rectally. Also I suggest that 99 rectally is a normal temp. I suggest that you're putting too much emphasis on his body which can cause him to become overly concerned and hamper his ability to live life normally.