Are you sure those aren't two blue and two black? Just checked, if you are talking about the computer in your signature; then there are indeed four slots supporting up to 2GB of RAM as shown below.
System Micro ATX HP Pavilion a-series chassis
System dimensions: 15.75" H x 7.5" W x 15.75" D
Chipset i915GV
Front-side bus (processor dependent) 533MHz
Power output wattage 300 Watt
Memory Total memory slots: four DIMM
Maximum memory expansion: 2GB
Expansion slots Three PCI expansion slots (two available)
Drive bays Two external 5.25" (1-2 occupied by CD/DVD drives, depending on configuration)
Two external 3.5" (0-2 occupied by floppy drive or media card reader, depending on configuration)
One internal 3.5" (filled by hard drive)