The Pentium 4 (also known also "Willamette") is, you guessed it, the follow-up to Intel's Pentium 3 processor. It's generally faster than its predecesors, and employs different technology in ways that are meaningless to your average consumer. All they need to know is "It's better than a P3!"
More than just a faster processor, the Pentium 4 clearly demonstrates the current lack of imagination in brand names. Remember a few years ago before everything was simply a functional description of the product followed by a version number or the current year?
Apple had Quadras and Centris's, We had the Alien, Aliens and Alien 'cubed' movies. Creative version naming! Now we have G3s and G4s and Fantasia 2000. Where's the fun in that?