The blue wire is for the trailer brakes in a larger trailer. This wire becomes hot when you hit the brakes. Since you are only using four wire harness you can wire nut and tape up the blue wire .

Check out this link... gives you the different wiring diagrams.
Trailer Wiring Diagram For 4 way, 6 way and 7 way circuits

 

Thanks everyone. I found a trailer wiring harness for the 2nd gen's, so it had a schematic. Got the hitch on and the harness soldered in about 20 minutes.