Douglas-Peuker and equal distances
I would consider the curvature in that case too. If the curvature is small - then it is a flat region - so you can safely remove it - your 4th point for example. If the curvature is large (your 5th point for example), even if the distance is the same, you should most likely leave it alone.

I would actually recommend a curvature based method coming before Douglas-Peucker.