We started potty training my son around 4. We ended up buying a potty urinal. That sparked his interest in potty training. We also had his preschool helping us at school while we were in the process. He took much longer to wake up dry at night. We found out that he didn't like being wet. He wore underwear and we did away with pull ups.
For my middle child, I ended up putting a diaper inside the potty chair. (She was 18 months and going through the process with her older brother.) She was afraid to go on the potty without a diaper. Once she was consistent I started "forgetting" the diaper in the chair and she didn't it without.
With all my kids, when we were training they were naked. No pull ups, diapers or underwear for the first three days except at night.

There is a video from bear in the big blue house that is about potty training. I used it for my grandson and it took quite a while but he finally got the idea. He was around 7 when he finally got fully trained.

Yes - it's very normal with an autistic kid for it to take more than four years to potty train. Just keep reminding yourself, it *will* happen.

In our case what worked was putting it on my daughter's plate. In a relaxed moment, I asked her, "when are you going to be potty trained?" She thought about it a bit and named her next birthday. And said "I'll be potty trained." So I didn't bother her any more at all about it until her birthday, the day after. I reminded her what she had said using her exact words. She didn't say anything then, but that was the last diaper I changed.

I'm having the same issue with my daughter!!