Two at a time

You have 8 stones, 4 yellowe and 4 green, arranged as shown below. By moving two adjoining stones without changing their sequence, rearrange the stones so that all the green stones are on the right and the yellow stones on the left. Try to achieve this in 4 steps.

Problem from:
van Delft, Pieter and Botermans, Jack. 1997. Denkspiele der Welt.
Heinrich Hugendubel Verlag, München
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