Trial 4 is the Jib Tricks Trial. The board and character selection does not really matter for the trial so just pick what you like. Prior to starting the event the game will run you through each task and give you a diagram of how to do each of them. In Trial 4 you need to do the following in a single run:
"Keep your balance while you're grinding, or you will fall." If you are having trouble, an easy way to do this is to just start the event and get out of the camera's view. You should land right on the first rail and ride it the whole way to get your check mark.