the positive lead from battery to battery FIRST and then the negative lead to ground points away from the battery. The point is to have whatever spark occurs when the fourth connection is made happen away from the top of the battery where there may be a concentration of explosive battery gases from the chemical reactions connected with it discharging. If you haven't had a problem (i.e. serious injuries or damage) with this yet, it's either because you're lucky or have only jumped cars which haven't had their batteries subjected to any current draw for some time, so there were no gases nearby.

Unless, of course, you're jumping old British cars or Fords from 1955 or earlier. My son has just acquired a '64 Sprite to restore, and one of my colleagues has a pristine '55 MGTF which I have to jump start most of the times she wants to take it out for a little exercise, so I have to keep that constantly in mind.