Ummmmm, do the sandwich maker test with new shoes and the old shoes (try and bend the toe to the heel). Then tell me you reckon the 4's are less flexible.
I've got both types too, and there is no way the 4.0's are less flexible than the 5.0's. They've got way deeper flex grooves, and less midsole material. Also, try and bend both shoes laterally (through the midline) - the 4.0 flexes significantly - the 5.0 doesnt.