Sounds like she is pregnant. Whether you separate her or not depends on a few things. First do you want to keep the fry? Second, do you have enough filter media to seed the 4 gal
tank for her? Otherwise you will have to do probably daily water changes on the tank. You will still have to monitor the parameters even with seeded media mind you. Do you have a heater for the tank? What arethe other tank mates and are they places for the fry to hide?
Mollies will eat their own fry so if you do separate her you will have to remove her once she finishes giving birth. Also do the move sooner rather than later as moving a very pregnant molly can cause stress and sometimes health problems as a result.