Debra,

The issue of  Rpm has a couple of components that affect the amount of
wear the engine is exposed to.

#1 At 1000 feet per minute of piston travel, or 4000 RPM  the rings on
the piston can begin to vibrate. or resonate.  The wear would be to
pound out the groove the keeps the ring in its place on the piston.
 There is some latitude, depending on the type of rings and cylinder
combination, but as you mentioned the VW green range is up to 4000 RPM.
 The hashed green range, would mean to me, an ex military pilot, a
transitent range to used briefly, say passing, but not a "cruise".

#2  The  other high rpm issue is the crankshaft.  Whether 1.9 or 2.1
WBX, there seems to be a concern of  wear on the center crankshaft
bearing surface.  This wear is attributed to the crankshaft flexing in
the middle, ends stay put. The effect is to elongater the center bearing.

sincerely,

Mark Keller
91 Carat