Warm/hot sensation on skin of my shaft, no discharge, visible rash or anything. friction burn from masturbation, or std from 4 seconds of oral sex?

Not an STD: No STD causes symptoms like this. And oral sex is safe sex -- not totally risk free, but STD risk much lower than for vaginal or anal sex. And very rare after such a brief exposure. You're the best judge about whether it could be a friction burn from vigorous masturbation: you were there, I wasn't! But if the skin looks normal, probably nothing wrong and the feeling should go away soon.