a)I need to store 4 bytes of data in the 89c51rd2xx in the non volatile ISP flash.
As per the data sheet Philips have some new functions :
Program Data Block / Read Data Block which should probably be able to do the same.Would it be possible to do the same with these functions?
b) Since this task is to be carried out by the running application program , it has to be done without pulling PSEN low by jumpers etc. Would this be possible ?

a) No, it is not possible to store just 4 bytes and use all the rest of memory for other purposes. You will need to erase a page of flash and store the 4 bytes there. Flash can only be written to a '0'. To get all '1' you must erase and erasure can only be done for a whole page (the datasheet will tell you the size of a page).
b) The 'pulling PSEN' has nothing to do with writing flash, it is a means of 'tricking' the processor into ISP mode.

Erik