Didn't you have to agree before your kids could be adopted (I presume they were adopted by your ex's new husband)?
Nonetheless, it sounds as though she is guilty of fraud. What struck me about your post, however, is that she did not allow you to see your kids for four years. Four long years. This could not have been healthy for your children, who must have felt deserted, and will never understand the manipulations and animosities of adults. This could not be easy for you.