1. Kumliens Iceland. Markings too restricted to wingtips and not dark enough for thayers.
2. Kumliens Iceland
3. Adult Herring Gull. Pale eye, yellow gape, and stocky build rule out California.
4. Which one is the possible thayers? Edit: my mistake. Looks like a dwarf Herring gull. I say dwarf herring gull because it looks slightly smaller in the photo which might have something to do with the individual. The head shape does look interesting but I think it's just due to the angle of which the bird is looking. The head looks too stocky.
Edit #2: I've put the reasons that lead me to each of the bird's ID above.