I think Tim might be wrong but I’m not 100% sure. Many years ago I used to run a flexo factory and yes we printed labels but not crack-back. All crack back labels were printed litho( if I remember correctly) as these are sticky back labels with a backing that has cracks in it to make it easier to get the backing off. The gum backed labels are printed litho on sheets and trimmed to size, similar material to a lick and stick stamp but you wouldn’t want to lick 100’s of A4 labels 😮
I would contact a paper supplier and find out if it comes in A4 sheets or only large sheet for litho presses.
things may have changed in the last 10 years since I ran the flexo department so could be wrong.
cheers
Warren