Hi there. Oh goodness, you have your hands full with a 4, 3 and 1 year old!! Hats off to you as a busy mom and I do hope you get a break once in a while.
A couple of things------ the potty training is within normal by the standards that my pediatrician gave me. Kids potty train at different ages and with different motivational levels. My second son was very resistant to potty training and did so very late. But really, most kids do get it and are fine in this department.
Her lack of talking and interacting is more of a concern. How is her speech? Has she developed normally in this area---- using lots of words when she does speak, articulating well, can answer questions back and forth with someone, uses full sentences? If speech is delayed at all, it can make a child withdrawn socially and otherwise.
How does she do with other things? Does she make eye contact or play with her siblings at all? Kids of three are often still doing parallel play which means they play alongside each other but not really with one another--- but they will talk to and interact with other kids typically. Does she pretend play with her toys?
I think the place to start is with your primary care doctor to get referrals to others that might be able to give you some idea if she needs any extra help. A speech pathologist would be good to evaluate her and perhaps an occupational therapist. My own son had some speech issues but also, social skills didn't come naturally to him like they do most kids. We had to teach him how to interact.
Keep in touch and let us know how it goes!! Good luck
Yes It does sound as if she needs some help with her speech and communicating.How does she get on with the other 2 siblings does she get the same attention as they do, do you talk and communicate with her in the same way ?
Since you have two other children that are problem free, I doubt that your daughter's behavior is any fault of yours. I would have her evaluated and get some expert advice on how to help her.