Sounds like a great idea as long as you realize that ultimately, the chicks are your responsibility. In my experience, even the most mature 4-year-old won't get the idea of feeding, watering, cage-cleaning, checking temperature and so on and so on 3 or 4 or 5 time every YES EVERY day for weeks and then several times a day for years. Also, a 4-year-old might over handle chicks during their first few days and that can create problems too.
My daughter saved up all her money for over a year, starting with her birthday money when she turned eight and when she had enough, she purchased a puppy. Now she's a great kid, good grades, generally good attitude, but although she considers the dog hers, the dog considers itself mine because I'm the one who most often feeds her and walks her. My daughter is quicker to take the initiative to walk her or bathe her than her brothers do, but 7 years later it is still usually up to me to remind her.
Good luck and post pictures. If there is anything cuter than baby chicks it's little children with baby chicks!