@AndZyk - Thanks for your reply,
the 'getRandom()' method is well known, but I think with your solution I only get 4 randomly choosen ALBUMS and not the desired 4 IMAGES out of ALL ALBUMS.
But maybe I didn't grasped it right ...
@szabesz - Thank you for the link,
I've already seen this post but was a bit distracted by the JS an slideshow discussion in there. Maybe I can extract some parts to solve my problem.
As far as I can see, a possible solution is to randomly choose a subset of 4 albums and in the next step select 1 ( only one) image out of all the images in the image field of each choosen album - hmmm ...
Any further tips?
Regards, Jochen