Best of all, that your OS and programs is located on a part "C" on hard drive. Because this part is most quickly. Other HDD's parts is located nearly to center of disk and by the constant speed of rotation of disk they are slowly permit to read data from itself. Also, defragmentation, (My computer -> disk C-> properties -> services -> defragmentation ) can keep constant velocity of reading data from HDD. Make it regularly, when comp want to do it.
The existence of 4 parts on HDD don't influence on speed.