Most female mbuna hold fry for about 21 - 28 days, so if it has been 4 weeks, it is safe to strip the fry from her. In fact it sounds like maybe you need to help her. I have had females that refused to spit and the fry literally ate through the bottom of the females mouth.
Get your self a 5 gal bucket and fill it with some water from your isolation tank. If you are right handed, then get your left had wet, hold her in the palm of that hand, and gently pry her mouth open with your finger nail on the index finger of your right hand and swoosh her in the water. Repeat until you know you have all the fry out. You will have to look inside her mouth. If you can't get her mouth held open with your finger, then try using a cap to an ink pen, a bobbie pin, q-tip, etc.
Don't hold her out of the water too long, and let her rest in between the process. Then put the fry in your fry tank and the female back in the main tank, or if you think she needs to recover a bit, put her in a tank by herself.
A 5 gal bucket works well, incase your drop her, she will fall back in the bucket. When you get better you can use smaller buckets!
You can do it!!!