hi joyti raina,
splitter —> technically speaking divides the power of input equally at the output.
i.e : if 10mW input is given to 1:4 splitter , the output power will be 2.5 mW (10mW/4)each
fork —> technically speaking replicate es the power of input to each of the output.
i.e : if 10mW input is given to 1:4 fork , the output power will be 10 mW each.
thus you need to use them as per the design needs
with regards