Question
I have started rhubarb from seed indoors.  The plants are about 4 months old.  They are quite hardy.  I have them in a large containers on my patio.  I was wondering -

How to keep them over the winter, so I can transplant them to my garden in the spring.  

Answer
If you have not had a hard freeze yet, you can just plant them directly into the garden and when the tops freeze, just cover them with a good layer of mulch.  Keep them moist during the winter and they will come back in the spring after developing a good root system this winter.

If you have had hard frosts already, I would overwinter them in a basement or garage, keeping them watered, moisted, but not too wet, and then plant them in the spring.