If it's a half-wave dipole, why would you use a 4:1 balun with coax? You would only use a 4:1 balun to match an impedance that's ~4 times higher or lower.
What you want is a good current choke, which is effectively a 1:1 balun. It will keep undesired currents off the coax shield, and keep the shield from picking up RFI.
I highly recommend that you read the excellent material on baluns and chokes on K9YC's site at http://audiosystemsgroup.com/publish.htm