I have a 4 month old beagle, and she sleeps on the sofa through the day and her bed at night, she has just started wetting herself when she is asleep. She doesn't wake up.

Has she recently been spayed? Sounds like she has a bit of incontinence. A lot of female dogs suffer from this at one point int heir lives. Some dogs grow out of it as their balder develops and some never do. If it is something like spay incontinence it can be treated with a medicine to help her with bladder control.

Just to add to Bella's post. I'd get her checked to rule out a bladder infection.