When you say a BOB for a family of 4 do you mean one bag for everyone or one bag for each member of the family. If your talking about one bag for the whole family then your going to need something along the lines of a large duffel bag or maybe a large plastic storage bin. If your talking about one bag for each make sure you get the right sizes for each member, they actually have sizes just for children and women, the choices are endless, and so are prices. Ebay is a good place to pick up new and used backpacks and duffel's for a reasonable price. I wish I could narrow it down for you a little more but your question leaves a lot of options.