I have a 4 year old who is severely speech delayed (didn't speak at all until 3 and a half) and has autism.
I got him trained day and night at 3 and a half. He was late, but he got there and he's been fab ever since - no accidents.
It was hard work. I worked out that he would do a wee around 20 minutes after having a drink. So id sit him on the potty after about 15 minutes with my iPad or a book and waited for a 'wee wee' and when he went I yelled happily praising him. He loved the praise but would wee himself if I hadn't sat him on the potty.
In the end, I let him roam the house half naked and would encourage him to sit on the potty even if he didn't go I would ask if he needed to go.. If I seen him start to go I would quickly run him to the potty to finish it off or to remind him where he should go..
After doing this for a while, he started weeing and pooing without being reminded BUT only if he didn't have trousers on which was a nightmare as he would pee himself with trousers on..
I used to sit his favourite toys on the potty to do a wee wee. He had a toy bath duck that filled with water in the bath. I would squeeze the duck over the potty to show my son that his duck went wee wee.
In the end, I just binned the pull ups as he would never tell me if he needed to go if wearing one.. I put him in pants thinking that id be constantly changing his clothes but surprisingly he seemed to get it and has had very few accidents since and none at all now.
I kept a potty in my kitchen and upstairs. Also another in the car to remind him when to go..
He went from barely being trained to trained really quickly considering he was speech delayed.
Good luck :)
Summer is a good time to let him run around without trousers on!