When you say, "slipping," please describe that a little more. Is it a smooth slip or a very grinding, rough slip?
Never had a WF4 but if it is a Magna Pull recoil, like a KD4S or KF5 Mercury, make sure the grease on the flat plate where the magnetic pawls travel is free of heavy or dried up grease. Just a little 105 or oil works. These recoils use actual magnets to create the moderate friction required for the pawls to move in and out. It doesn't take much to mess that up. To check its basic operation off of the motor, you can place a good size socket in where the pawls go in and out, to pull the recoil out without the pawls over extending and locking the mechanism. And make sure to check a parts diagram to make sure you don't have washers where the shouldn't be, shimming up the fuel tank too high, making the recoil ALSO too high. Make sure also, that the motor hasn't been dropped or otherwise mishandled, bending or breaking the brackets that hold the fuel tank to the power head.This would allow the recoil pawls to ride up and over the teeth of the flywheel nut/ratchet.