Hi C.,
Posterior fontanels should definately be closed by the time the baby is 4 months old. The fontanel on top of their head should be closed by the time the child is btw 12mo-2years.
I don't want to worry you but yes, it is a big deal if the posterior fontanel is not closed. The doctors will probably want to do an MRI just to take a closer look at things and then go from there. It's great that she is developmentally on target! Just protect her head if she falls backwards or anything of the sort. Place some pillows around her if she is trying to sit up right now. My little one was sitting up at 4 months so some do start early.
And one last thing...your last response regarding praying and waiting...well it's the best thing you can do. Try to leave it in God's hands He loves your little one so much more than you could ever imagine!