I think the rationale is that you select S mode because you want exta oomph. Therefore 4th gear is a reasonable starting point for most situations.
You need to bear in mind that S mode isn't a true 'manual' mode. It just allows you to specify the upper gear that the auto box can select. Consequently, its only use is when you want to select a lower gear than would be selected when in D. i.e. when you want to push the car that bit harder.
To be honest, I can't say I've ever found a use for it. If I want extra power I just floor the throttle, and I mean floor it. There's an extra bit of travel at the bottom of the pedal range that requires you to push it down harder. That normally takes care of my extra power requirements when overtaking.
Hi, I use manual mode to support brakes (downhill, traffic-lights, emergency braking).
I expected that the driver uses manual mode to "influence" the gearbox - therefore I would expect that chanigng from auto to manual do not influence engaged gear.