That's a long haul. I would still use conduit. You will need to calculate the voltage drop vs amperage draw. Your wiring will need to be upgraded to account for the V drop.
Roll up a ball of duct tape into a firm ball that it is about 1/8" - 1/4" smaller than the ID of the conduit. During the ball construction, make sure a very lightweight string it rolled into the ball to attach it. The string tail has to be over 400 feet. Also, make sure the ball exterior is NOT the sticky side of the duct tape (or whatever type of tape you select)
Use a strong shop type vacuum to suck the ball of tape thru the pipe. When it gets to the other end, attach a stouter pulling rope to the string and pull it back the other direction so the pulling rope is now inside the 400 feet of conduit. Attach the wires to the pulling rope and start yankin'