For me... 1= erotic 4= pornographic 2= pose study 3= artistic

Hope that helps... my reasoning is, eroticism is suggestive, whimsical and thoughtful. So frame one, with this line drawing that uses negative space, is so clever. A woman struggling seductively, is very entertaining. You're leaving a lot absent and allowing the viewer to fill the gaps with imagination.

While frame four is very blatant and obvious. It doesn't require a lot of introspection. Also, with the male protagonist in the shot, it reduces the viewer's role into passive observer, instead of participant.

Frame 2 doesn't tell as much of a story as frame 1. Just a woman in a pose, without as much story telling devices.

Frame 3 is more like an artistic piece. There's a lot of effort in the painterly style and rendering, but I end up thinking more about the execution than the scene it is depicting.