The BEST solution ......

When you pulled in the field on Monday and saw the volunteer corn, you should have pulled the planter out of the field immediately before planting, and hit it immediately with Select. Then wait to plant.

At this point, if you are going to use Gramoxone/ Paraquat on it, I would wait and watch... allow the planted corn to start the germination process... allow it to get a root started.... then hit it with Gramoxone just before it is ready to emerge, or even just as it is spiking.   This allows the volunteer corn to grow a bit more.  I think hitting the volunteer corn at 4" will just burn it off, and not kill it. I believe the volunteer corn growing point is still below the surface.  To kill it with Gramoxone, you need to have the growing point above the soil surface. Waiting those couple of days will give you a better change of killing it. IMO. The risk is ... it gets wet and you can't get it sprayed.  At that point, it isn't the end of the world - you will just have to live with the volunteer corn.  

When you spray with Gramoxone, add a bit of atrazine with the Gramoxone.  Even 1 pint/acre would beneficial.  (And be sure to use a surfactant with Gramoxone - that should go without saying.)