How do you fix a cracked finder on a four wheeler?

Order a new fender from your dealer, it is the easiest way to go. I have repaired a few with a drill and stainless tie wire. First, drill a small hole at the end of the crack then a few down each side of the crack. Then, use the wire to lace up the crack.

They do make plastic welding kits now to repair auto plastics used at body shops. It's about $60.00 for the kits to start. Then just buy refills.