Welcome!
Perhaps you could clarify whether he retains bowel movements in general, or if he is reluctant to have bowel movements on the toilet, as those are two separate issues.
Just turned four is not too old to be using a pull-up for bowel movements. (There's no rule that says everyone has to be in underwear day and night at any particular age!) If he isn't comfortable with the toilet yet, I wouldn't push it to the point of retaining his bowel movements for excessive periods of time, nor do I view it as worthy of punishment if he soils himself because he doesn't like the toilet. Keep in mind that using a toilet is not a physically intuitive posture. (In fact, quite large portions of the world outside of North America and Europe find our sit-toilets rather peculiar and unsanitary.) He clearly is perfectly capable of using a toilet, but uncomfortable with this particular toileting convention.
The straightforward solution would appear to be to put him back in pull-ups for defecating. He still has time to get used to toilets.
