HELP!!! New kitten is urinating on carpet

If you read my earlier post it tells about me finding a little kitten along side the road a few days ago. She's so sweet. We named her Allie. My four older cats are slowly coming around (no cat fights so far). I've been bringing her out a couple of times a day to slowly introduce her to the others. My youngest cat Lily has even beeen playing with her a little today. The problem is I have caught the kitten urinating on the carpet a few times. I have her food, toys and litterbox in my bathroom. At night and when I can't watch her she stays in there. She does use her litterbox, but if I have her out with the rest of my pets she will go to a corner urinate & then dig at the carpet. She hasn't had a BM outside her litterbox though. I originally thought that she was maybe 9 or 10 weeks old and just very tiny for her age (1.4 pound). The reason I thought this due to the fact that she doesn't have blue eyes. After looking on the internet though it says that their eyes can change to the adult color as early as 6 weeks. So maybe she is only about 6 weeks old or so. All four of my cats came to me littertrained. One of them was an adult when we adopted her, two of them were older kittens (about 10 weeks old) when we adopted them, but my one cat Sabrina was brought home from the shelter at just 6 weeks old and also was very tiny (1.5 pound). We kept Sabrina in my daughter's bedroom at night and when I was at work for the first few months (just like this one) and she never had any accidents outside her litter box. She was an owner turn in though. Apparently the people turned in three kittens but kept the mother cat. She was suposedly (sp?) an indoor only cat that had escaped just once (long enough to get pregnant). So I guess it's possible that the reason Sabrina was litter trained at 6 weeks old was that her mother taught her? The poor little kitten that I found for all I know was born outside and may have never been taught be her mother how to use a litter box Any advice? I thought that all kittens just basically litter trained themselves. I have been keeping her shut in the bathroom when I can't watch her, so hopefully she gets the idea and starts using it regulary. My husband is already less than thrilled with the idea of a fifth cat. If she keeps this up I'm afraid he won't let her stay. I'm trying to stay calm about it and just give her some time to learn. Maybe if I just keep her in the bathroom most of the time for a couple of weeks she will get the hang of it. One site that I was on said that kittens that were born outside sometimes think that the carpet is grass and that's why they prefer that to the litterbox. It suggests trying to keep them away from carpeted areas until they get in the habit of using the litter box

Try placing a litter box out in the main part of the house, a little closer for her to get to ... sometimes, little ones just can't hold it and have accidents.

and because she is so tiny use those disposable aluminum foil trays- they are cheap, you can get a lot of them, and tiny bodies can climb over the sides easily