It's just missing a few steps: First, remove the end from the lower (still empty) half of 7-minute hourglass, causing it to drain its sand, while it runs. When the 7-minute hourglass runs out, do the same with the 4-minute hourglass (being on its second run). Flip the (now empty) 7-minute hourglass over and let the rest of the 4-minute hourglass drain into this. As the latter runs out, flip the 7-minute hourglass for 1 minute worth of accumulated sand (minus a few grains lost over handling procedures). ;-)