Oh, I need to add that you might need to be VERY vigilant with your dog and young stock. If she was not brought up to respect babies, she will have to be taught carefully and closely supervised the first couple of years she'll have youngsters in her charge.
And don't be disappointed if it doesn't work out - it's possible she may never learn to treat little ones gently. It's frustratingly difficult to reverse the process from a pet, being used to play any way she wants, to having to control her impulses around babies.
It doesn't mean she's mean, or a bad guard dog, or that a guard dog wouldn't work for your situation. You just might want to consider raising a pup to guard your lambs, if your four or five year old dog doesn't work out for that job. Guard dogs are happier in pairs, anyway.