I would say the 4L should be fine but tight (which is probably good). I have a 3L Camelbac and it fits decently in my Timberline hydration pouch with some room width wise but not length wise. Do you want a measurement?Dromlite. The original dromedary bags taste awful.. Google it I'm not the only guys that thinks this... : ) The better question is what size Dromlite. I have a 4L I look forward to testing when DT1 comes in... I also keep the original cap in my bag (the hydration kit screws onto the lid). Its nice to have the option of using it without the hose.