Most children are indeed toilet trained before age 4 and most are toilet trained at around 3 years old and sometimes as early as 18 months of age.
When the child turns 4 and is still in diapers all the time and not using the toilet at all then you really need to force the issue on potty training because children can't start preschool or kindergarten when they are still wearing diapers full time unless there's a legit medical need for the diapers.
Some children are not ready to toilet train until 3 years old and if so that's okay too since all children toilet train and get out of diapers at different ages.
My son didn't want to toilet train until 3 years and 6 months old and I tried before that at 18 months old but he wasn't interested in the toilet at all so I just waited until he was 3 years old and tried potty training him again and he toilet trained much easier.
So if your child is not interested in the toilet before age 3 then it's okay and you should just wait awhile and put the child back in diapers and then try a year later when they should be ready.
I wouldn't worry about toilet training until the child is 4 years old and still wearing diapers and then you really need to force the issue again because children can't start preschool or kindergarten in diapers.
Keep in mind that when you begin toilet training the child it can take up to 6 months to a year even before the child is fully potty trained and not having accidents so during the night or when you're away from home or on a long road trip then the child should probably wear training pants or even diapers until they are fully potty trained.