What are some things that come in fours that I can use for a coaster design idea?

I have four coffee table coasters that I want to apply vinyl graphics to. I want to do a theme with 4 different images that work together. i.e. the 4 teenage mutant ninja turtles (although I don't want to do them). I'm open to lots of different ideas; cartoons, movies, videogames, music, nature, etc. I've been trying to come up with a good idea for awhile but it's just not happening and I'm getting tired of looking at boring plain coasters. Thanks in advance!