these are mostly 2nd cycle herring gulls (pics 1 and 4, 2 is prob. a 3rd cycle herring gull),
just the one in pic 3 could be a caspian gull but it has a lot of head striation and thus indicates at least some argentatus blood - if not a 3rd cycle argentatus itself...
Maarten,
I am sure that, if you put less emphasis on such intangible things like 'structure' and 'jizz' for now, you will find that these immature birds all have the typical plumage of 2nd or 3rd-cycle Herring Gulls.
Just compare with field guides, Olsen/Larsson, or Ruud's website, for instance.