Recently, my 4 year-old daughter goes to the toilet too late and sometimes soils her clothing before she can take it off.
I don't know if she deliberately postpones urinating until the last possible moment, or is not aware about it soon enough. I think I should take her to a doctor if it's the second one.
I tried to ask her but she does not understand what I'm trying to ask. Have you encountered a similar situation?
I would suggest you make a routine with your child to go to toilet more often, even if he/she says it's not needed. Often just sitting there makes it happen and creates a nice feeling of relief.
This way the whole subject will receive less importance which is often the source of the problem taking so long time to disappear.
I agree with this other answer here and that's exactly what we are doing with our own daugther and this is obviously helping
As for the original question:
How to tell if a 4 year-old child deliberately postpones urinating until the last possible moment
From long time of observing our daughter, she has the same behavior as adults who hold their urination for long time: crossing her legs, touching her backside etc.
While searching I found a good article in this matter:
Children who have not developed brain control over their bladder may try to stay dry by contracting their sphincter muscles at the same time their bladder tries to empty. Sometimes a child either crosses the legs or squats down when trying to hold back urination. In most children, the reason they develop the pattern of contracting their sphincter muscles at the same time their bladder contracts is unknown. Unfortunately, once children begin this pattern, it is difficult for them to learn to relax these muscles when they try and urinate. This abnormal activity of the sphincter muscles occurs in most children with voiding dysfunction.
So I tend to believe this is the reason, not deliberate holding.
It is common at this age for kids to be so engaged in their play, they deliberately hold it until something more boring comes along. Then, they forget they need to go until it is too late. (I taught preschool, and had the class of kids that were having accidents or were "in training" (they called me the potty wizard I mostly had 2's and 3's but had an occasional four year old that had "reverted" as well).
Since they don't fully grasp cause and effect at this age, it can be a stretch to get them to remember or explain why they waited or why they didn't go in the first place. This would explain your daughter's inability to answer you when you ask.
I wrote a detailed answer about how to deal with it on this question that offers more detail about how to talk to her about why it is important to go so often in a way that will be meaningful to her.
Whether the reason is Partial holding (she goes potty, but then doesn't let all of it out and as a result needs to go again sooner than she otherwise might need to, or simply that your daughter is enjoying play time and doesn't want to interupt it to go potty, the response is still the same.
Make it routine to have her go use the potty frequently whether she thinks she needs to go or not. Every hour to hour and a half should generally do the trick. I also have my current charge (three year old boy) go potty any time we change rooms or activities. That means in order to move from inside to outside he must attempt going potty and clean up the current activity. He goes before nap (even if wearing a pull-up - it is good to practice), he goes before we go outside AND he goes when we come back inside. It is so much a part of the routine he rarely protests even if he has gone ten minutes prior but now we are about to go on a walk. Again, for more details you can look at my answer to this other closely related question.
If the problem continues, asking your pediatrician about it at her check up just before she turns five, would be a good way to reassure yourself there is not a physical problem, or to confirm what the problem is so you know how to address it and move forward.
