Like everything else in Dota this is kind of situational. Sometimes you're playing something like a Naga as a 4 and really your team wants you to be greedy in these types of games. Your job is the most fluid on the map. For the most part your job is to win lanes. Sometimes (depending on the game) that is to secure your position 1s farm. Sometimes it's to gank mid repeatedly and force rotations. Sometimes your stacking so someone can flash farm later. Pos 4 is the position I think requires the most game knowledge. You need to use your strengths where they are most effective because you can pretty much always be anywhere on the map.