Does the separating boundary of a given set of points *have* to be a straight line (or a flat hyperplane)? 

The image below seems to be clearly linearly separable.

[![enter image description here][1]][1]

The other one here (the classic XOR) is certainly non-linearly separable.
[![enter image description here][2]][2]

But how about these two?

[![enter image description here][3]][3]
[![enter image description here][4]][4]

Both of them seems to be separable by a single line, though not straight. Are they linearly separable?


  [1]: https://i.sstatic.net/fFaAp.jpg
  [2]: https://i.sstatic.net/Pg2VH.jpg
  [3]: https://i.sstatic.net/gucSB.png
  [4]: https://i.sstatic.net/dE34A.png