Read potty books.
Buy potty with a baby and let baby sit on it.
Gradually take off diaper and use skinny underwear.
Give m and m for pee and poo in the potty.
Mine still have diaper for going out and at night at 4. I'd say start at 2.5 -3.
Most are ready between 2 1/2 and 3 1/2. The PP is right. Just do all those encouraging things and you won't have to train. I know that is hard to believe, but if you talk it up once in awhile, let them see you go, buy them some cute undies, etc. they will literally let you know when they have to go. Our oldest just woke one morning and said mommy I have to go potty. That was the end of it. He was trained that day. Our ped gave us that advice that first time and it worked for all three of ours.
The best advice that I ever received was to get them used to the potty at a young age before they have a chance to develop any fears about it (I received this advice from my mother-in-law who raised 9 children). I noticed that my second daughter was dry in the morning at the age of either 1 or 1 1/2 years of age. What I did then was get her out of the crib and immediately go to the toilet (with the insert on it) and put her on it every morning. She peed in the toilet every morning and when she was 2 she started to tell me that she had to use the toilet. She started out telling me maybe a quarter of the time at first. She continued to wear a diaper and I would just put her on the toilet when she told me. She eventually worked her way to telling me all of the time.
This was my second child. Of course I was scared to death with my first child and I messed it up big time! I didn't have any anxiety with my second child because I "learned" with my first "experimental" child. Sometimes I wonder how my first survived!
Wow 4 years sounds a bit old, although i accept all children are different. My daughter was 2 1/2 and i was told that was late. Here in the uk children are required to be trainned before they go to playgroup at 3 years. My daughter wasn't easy ..I used "star charts", I made a poster for the bathroom door and every time she was successful I gave her a sticker to stick on her poster. If she got a certain amount of stickers in a set timeframe she got a reward. if she didn't i just didn't make a big deal of it. Using this method she was trained in a weekend...we have never looked back ........good luck.
Potty training is a nightmare for me. I have a 4 year old. Unfortinately when she was ok with potty, we moved her to toilet to quicky and had great set back. Right now we gave her potty back and diapers at night.
She started going on potty but having too many accidents as well. And she absolutely refused to use public bathroom.
She has to be fully trained by 5.5 and I am getting desporate.