How do you wire a 3 prong 240 volt spur to a 4 prong 240 volt receptacle?

Your L14-30 receptacle has four positions on it. It is rated as a 125/250 volt device. It is classed as three pole four wire grounding. Connect the ground wire to the "G" screw, white wire to the "W" screw. One of your hot wires (might be red) to the "X" screw and the other hot wire (might be black) to the "Y" screw. This connection gives you the potential of 240 volts between "X" and "Y" and 120 volt potential between "W" and "X" and 120 volt potential between "W" and "Y".