Regarding hiding Selection screen block

hi all ,

i have 2 blocks in the selection screen. the First block has 4 radiobuttons and the second block has 4 parameters.

i need to hide the block 2 based on the radiobutton checked in the initail block.

if radiobutton 1 or 2 is selected then the block should not be visible and if the rdbtn 3 or 4 is selected the block should be visible

i have wriiten the code but it doesn't work at all. the block is hidden but when i select the radiobtn 3 or 4 it doesn't work

the code i hve wriiten is as follows

SELECTION-SCREEN BEGIN OF BLOCK b1 WITH FRAME TITLE text-001.

PARAMETERS: par1 RADIOBUTTON GROUP rg2 DEFAULT 'X' USER-COMMAND ucom1

PARAMETERS: par2 RADIOBUTTON GROUP rg2 .

PARAMETERS: par3 RADIOBUTTON GROUP rg2 .

PARAMETERS: par4 RADIOBUTTON GROUP rg2

SELECTION-SCREEN END OF BLOCK b1.

SELECTION-SCREEN BEGIN OF BLOCK a1 WITH FRAME TITLE text-001.

SELECTION-SCREEN BEGIN OF LINE.

PARAMETER p_pwwrk TYPE plaf-pwwrk MODIF ID FSC.

SELECTION-SCREEN END OF LINE.

SELECTION-SCREEN BEGIN OF LINE.

PARAMETERS p_wch TYPE cr_hname MODIF ID FSC.

SELECTION-SCREEN END OF LINE.

SELECTION-SCREEN BEGIN OF LINE.

SELECT-OPTIONS s_mdv01 FOR g_mdv01 MODIF ID FSC.

SELECTION-SCREEN END OF LINE.

SELECTION-SCREEN END OF BLOCK a1.

I have declared this after Initialization.

AT SELECTION-SCREEN OUTPUT.

if par3 eq 'X' or par4 eq 'X'.

LOOP AT SCREEN.

IF screen-group1 = 'FSC'.

screen-active = 0.

MODIFY SCREEN.

ENDIF.

ENDLOOP.

endif.

can anybody please suggest.

Thanx

Srinivas