Hi, A..
Listen to what these gals are telling you. Please be mindful of the appearance of his stools. Small pebbles? Dry? Very dark? Smells strongly of iron? or Sticky? Tar-like? Adhering to his skin?
Pay attention to if he has spontaneous accidents.
Pay attention to his daily diet, just make a mental note on what you do see him eating or reaching for naturally.
Healthy stools - this goes for us, too - are long, unbroken and "snake like" (sorry to be so graphic, btw - not trying to gross anyone out). The stools of children are larger in girth because their colons are healthier and not lined with years of "gunk".
If he is a soda drinker, take it away (sorry). Don't worry about veggies if he doesn't love that, but please push fruit. Especially things with fiber. Things with (as my mother says "thread"). Mango (very thready), pineapple, celery, PRUNES (this will do double duty, obviously). The "fruit acids" (citric acid, pectin...) will help his digestion and "push" better stools.
If he likes veggies, avoid broccoli, try squash instead (again, "thread" qualities), soft carrots (like in soup broth), cucumbers are AWESOME - actually a big plus for your colon because of the high potassium.
Don't bombard him (too much acid for his digestion). But stay away from sugary drinks and white bread and CHEESES.
GRAINS and FRUIT: feed him like a bird (best way I can put it). If you bake? Throw some flax seed in with the muffin mix. Granola bars, raisins. Think "birds and squirrels"!
At 4 1/2, I am simply wondering if he's recoiling from the toilet for #2 because (like a cat with a bladder infection), that toilet seat represents "a pending painful experience".
I have two girlfriends going through this with their sons. One, her son is 7 years old and this has been going on since he was 3 (encopresis diagnosed too late and she is JUST NOW adjusting his diet!!!!!) and now friend #2, her son is regularly constipated with terrible gastric pains - he is 10; also - must now adjust his diet.
Try eating like a bird with him, MUCH fruit, MUCH water and try to incorporate very warm chicken broth in, too. Go for a walk (I know, it's snowing today but it's actually pretty out); walking will get the ball rolling (just like when we were all trying to expedite our labor "when it was time"). Let him run around and play in the snow and try to be nonchalant about reminding him to use the toilet; if his mind is a bit distracted he may not realize his habit of hesitation at this point.
Sorry to be so long winded and don't expect a miracle in a day (but you could see improvement and you might find some relief on your son's face). But please don't rule out bringing this concern to your pediatrician. Good luck, let us all know how it goes!!! *hugs*