Wow, $400/month for a family of 4 - including a teen and pre-teen - is very impressive! When our boys were that age, we swore it would be cheaper for us to pay their rent than their feed bill.
For DW & I - we average about $525/month for food, including the occasional meal out.
Very impressive! When I was between jobs 5 years ago, I was budgeting $800/mo for 3 of us. I know we could have done better, but DW just wasn't on board with the program. Not a spendthrift by any means, but also totally uninterested in the details of our budget...