[richappy]

Your new dryer may come with a isolated frame ground, ie, power ground not shorted to the frame.
If you just wire it up with a three prong pigtail, you will have no protection from an internal short, ie, you just might get a dangerous shock!!
For safety reasons,it's best to use the four prong pigtail. New dryers have wiring instructions for this printed on the back panel.
The 4 wire pigtails protect you in case of a lightning strike by grounding the frame to solid earth ground at a water pipe and routing the power ground to the power ground connection at the fuse box. One thousand amps running through ground during a lightning strike will elevate your dryer frame to 1000 volts, assuming one ohm resistance!!The four wire pigtail will prevent this.
To do this, you will need to rout a new dryer cord from the dryer wall outlet, to the fuse box.