What system model? Inspiron N5110, 7537, etc?
Four beeps is usually a memory error; three means a bad mainboard.
Unplug the system, remove the battery or disconnect it, hold the power button from 30 sec and remove and reinstall the memory module(s). If that does nothing, try one module at a time in each socket until you isolate the bad module (replace it) or the bad socket (which, in case of the three beep sequence as well, means a replacement mainboard).