The 4ltr carrier is a bag I believe. Currently with £10 off at the big walking shop in Skipton.
http://www.ultimateoutdoors.co.uk/item/Brand_MSRDromLiteBladder_0_0_33628_1.html
http://www.ultimateoutdoors.co.uk/item/Brand_MSRDromLiteBladder_0_0_33628_1.html