Linda LEE- Yes she says she is using Timothy.
You're in luck because I'm going through this. 4 weeks is a LONG time for a guinea pig to have these types of stools. Have you been weighing her? How does that look? Do they smell funny or are black? Any blood? Does she squeal when she goes to the bathroom? How about vit. C? with mushy stools I reccomend a good 50 mg (which is one tablet of Oxbow's vit. C supplement).
This warrents a vet visit. Get a few stool samples done because if there is any bacteria, parasite, infection.... the list goes on but you don't want it in her. And the reason for doing more than one, sometimes they don't find them right away.
Putting her on baytril in the meantime might help. Otherwise ask your vet about Metoclopramide, I can't tell you what it's used for but it was given to me.
Rice water (boil rice and take the water) works well for a day or two. You could also try getting some pedialyte.