to Preschool? or not to Preschool?

I have a dilemma- my daughter is going to be 4 in October, and her older sister will be 7 in October going to first grade, so obviously the younger is eager to go to school. This year was rough, just having the older in Kindergarten made the 4 yr old that much more eager to learn letters and numbers and colors and shapes, ect. SOOO, I am considering preschool for her this year even though that means she’ll have preschool for 2 years. My oldest daughter who will be in 1st grade only had 4 months of preschool and is doing fine in K, so I don’t think the younger ‘needs’ preschool for any huge reasons, just to challenge her and keep her mind up where she wants it to be. Get my drift? But I’ve also heard that staying home with mom helps kids feeling of stability and ‘free-ness’ last longer and is good for them. I don’t know. COMMENTS or SUGGESTIONS?????