To Capture the f4 value in selection-screen

Hi,

I have a problem with selection-option, when I am not giving any value in the select-option_low , then the value entered in the select-option_high has to be taken, i coded and it is taking also, but when i go for other value, the previous value has to be cleared and the current value has to take , but it doesn't happening, how can i clear the previous value, and also how can i capture the value when i press the f4 in the selection-screen.

Regards,

Narasimha