Some things I picked up along the way that may help the OP.
(I appreciate others may disagree)
Drag-Lifts
A tall adult on a snowboard will find it excruciatingly painful going up on a long T-Bar with a four year old. All the pressure will be on the side of one knee.
If it wasn't busy or on unmanned ones,, I'd put him on the one before me so if he came off, I could get off too and we'd go down together.
If it was busy, he'd usually get paired with a skier anyway and I'd go on the one behind again.
Pistes
I'd always make the boy ski in front. That way, when he crashed out, I could quickly snowboard down to him to help.
Walking back up isn't fun.
Losing each other
It's not likely but possible that you miss each other in a queue or there is a misunderstanding so give him a tag with your name and mobile number on it and attach it to his jacket. And then carry your mobile.
The lift operators expect children to have this information and will look for it.
Toilets.
Little boys get a bit carried away with all the skiing and insist they don't need to go when a toilet is available.
You'll end up washing out ski boots that evening.
So, drag them to the loo even if they say they don't want to go.
Clothing
Kids get cold - colder than you so ensure they're wrapped up warm.
Tiredness
Kids get physically exhausted before their enthusiasm wanes.
You may want one last run but they may be just too tired to be in full control and end up hurting themselves.