That's not quite as simple as you might think. How do you determine when to over-ride the playbar button funtionality so that it works this way as opposed to it's normal behaviour.
Do not use a default playbar if you have a non-linear course. You will need to set up custom navigation buttons, which is not that hard since several button types can be used on master slides or timed for the rest of the project.
I agree with Lilybiri - ditch the playbar and use custom navigation.
In the above example, you will be taken to the fourth slide in your deck.
0 = slide 1
1 = slide 2
2 = slide 3
3 = slide 4