Card 4 picks up a motif from Card 3: the postage stamp with landscape. The landscape is the same (at least sketchily so): one tree, far horizon, some mountains in the distance. In this case, there is also a sun. The landscape is repeated below the surface and is viewed through a postage stamp shaped hole in the ground. It is also repeated in the upright postage stamp being propped up by a slightly sketchy board — or is the frame transparent? It should be clear (the answer, not the frame). No big mystery, but people see what they see.