Sonar sensor questions!

Hi, I am working on a little robot for my arduino. I am going to buy a sonar sensor, but I found out that there were 4-pin and 3-pin sensors. Are they any different? if so, how? I really want to save a PWM pin for my RGB leds. Also, will this really work? http://www.instructables.com/id/Hack-an-HC-SR04-to-a-3-pin-sensor/?ALLSTEPS

with the 4 pin, they have split the data lines to Echo and Trigger, instead of a single line. The 3 pin code will work, you just need to tweak it to work with trigger and echo. HINT: one is input and the other is output.