I am so sorry, that must be a terrible feeling. If your family was really close when you were growing up, it makes me think that they would also be missing you too. If it were me, I think I would try to have a talk with mom. Get her side of the story and see how she is feeling. You might find out that there has been a misunderstanding or maybe even an entierly different reason for their coldness. If she is not going to love and accept your children or at least make an effort, than I would probably keep my distance, focus on your four wonderful children and befriend some older couples that can serve as "fill in" grandparents.
My parents were strongly opposed to me dating my (now) husband. I never knew that there would be anything that could make us be at such odds with eachother. It was shocking, hurtful, and heart-sickening. They have since overcome their prejudices and accept and love my husband and daughter as their own. It took a long time though and was not easy for anyone. Prejudices that you grow up with run deep, and as awful and wrong as they are, I think that they are hard for most people to overcome.
I wouldn't expect things to be suddenly fixed by you reaching out, but your mom has to at least want to make an effort. If she is not willing to do that to keep you in her life, than as sad as it is, you will probably have to move on. You have my heartfelt sympathies. Good luck and let us know how things go.