Something about your life made you admittedly unable to care for your children so you turned them over to your parents. Experience tells me that there are a number of possibilities and these usually include lack of employment or stable housing. I have been at this game for a very long time, and have dealt with a great many situations similar to this so I have a good deal of experience to draw from. Yes, it is inference and assumption, but you leave us with little else as you have provided no details.
So, perhaps you would care to enlighten us how things have changed over that time so that where you were once unable to care for your children you are now ABLE to do so? Now, after four years you have suddenly decided you want to play mom and have your kids back? What has changed? Certainly it had to do with you and your situation, unless you just decided life was easier without the kids. As such, what about your situation has changed for the better? Is it now stable? Are you absolutely capable of providing for your children at least as well as your parents have been?
Have you brought the police to the home to reclaim your children? If not, why not?