My son is autistic as well and potty training just seemed to take forever. He is almost 5 now, and yes... it took until about 4 to get him potty trained.
Those with autism often require very strict routines and schedules. Thus, you need to make this a stringent part of the routine. Try talking about potty training, that you will be beginning to try to go to potty and what the potty is for. I brought my son with me whenever I had to go. We would go at certain times of the day... when he first woke up, at AM snack, lunch time, PM snack, dinner and before going to bed. That is the schedule I stuck to for a long time... until now... when he can tell when he needs to go and does so on his own.
Thus, try talking about it for a week. I suggest buying books on potty training, download pictures she can color, tell stories about it, maybe take her shopping for a potty or training seat cover. I purchased stickers and candy to reward my sons with. At first, any time sitting down on the potty, reward a sticker/candy. Try that for a couple weeks. Then reward additional stickers for pee-ing, pooping, wiping, pulling up underwear, pulling up pants, using soap and water to wash hands. I made a huge deal with hugs, kisses, congrats and told everyone (make a big to-do) about going potty.
I bought like 200 stickers for about $2.00 at Wal-Mart. They were things my boys were interested in like sports, thumb up signs, stars, animals, holidays, etc. I also bought small pieces of candy such as DumDum suckers, sugar free suckers, bite size mini-Reeces cups, chocolate kisses, Sweet Tarts, etc. My girl friend would give M&M's. Another friend would tell their child... let's go to the potty and see if we can make bubbles. When they would pee, they could see the bubbles in the toilet. Odd... I know... but whatever works...lol.
It's really important to get everyone in the child's life to incorporate a strict and consistent schedule and encouragement of going potty... or really any change in the schedule. Transitions are difficult for autistic children. Often talking about big changes a week ahead of time... consistently... helps to ease the emotional breakdown and fits. They know what to expect and feel supported. Also, you need to talk about the event directly before trying to do whatever new thing it might be.
Many autistic children have occupationational needs, so lightly brushing the skin while doing a new event can help lessen stress. Some people also benefit from wearing a tight shirt or weights put into a shirt. The constant pressure and tension on the skin helps to reduce emotional reactions and reduces stress. Try the brushing and pressure clothing about 10-20min before the activity and see if that helps to cope with the stress and transition.