K.M.
I think this is an excellent endeavor. 4 is the perfect age to work on becoming bilingual. So much easier than when you are an adult! I have heard and experienced wonderful things at Smarta School. They offer both a bilingual preschool - speaking roughly 80-90% in Spanish the entire day - and tutoring services. You can find out more about them at:
http://www.smartaschool.com/
Tel. ###-###-####
They are super nice. The director, Martha, and all the teachers couldn't be sweeter. She is perfect with the kids, gets great results, and is very helpful. Give her a call... if you live too far away she might be able to find someone who can help you.
Good luck!
K.