We're both half-right.
Basically, it's not the number of prongs, it's the number and orientation that determines voltage and amperage. A four-prong outlet is always a dual-voltage, 120/240 circuit for the purpose I said above (I just had to install one in my workshop), but 30 or 50 amp depending on the configuration. A three prong outlet can be...damn near anything from a 15-amp 120v grounded line to an old 50-amp 120/240 line.
Given we're talking about dryers...it's an old 30-amp 120/240 line (three prong) with a new 120/240 plug (four prong). Plenzmd, can you confirm the shapes of the plug and outlet here? http://www.frentzandsons.com/Hardware%20Re...onfiguratio.htm