Have him watch you.  If he's almost 4 he will catch on pretty quick.  You may need to walk him through the steps of getting his pants pulled down, aiming his penis, and standing at the toilet with his legs at a stance where he is comfortable.  Unless he likes sitting, this should be pretty easy to overcome.

That is a simple one. Just teach him to do so and he shall acquire the habit. Might require some time but be patient. Doing it standing up is much easier than doing it while sitting.