If you are interested in sending all four children to study in Canada, a coed boarding school that offers grades 5 thru 12 will meet your needs. This can get expensive, however many schools offer financial aid and discount programs, so be sure to ask each school what rate they can offer your family. They will be willing to work with you.
Another option you may consider is a homestay. This is where the school will help you arrange a Canadian family for your kids to live with. Homestay programs may be less expensive, however if you want all 4 of your kids to live together, it may be difficult to find a family willing to take everyone in.
Niagara Christian Community of Schools and Venta Preparatory School are two more affordable boarding schools in Canada, however for a complete list, use the school search tool and search with the following information : Grade 5, Coed, Boarding. This will return an accurate list of potential schools.