Checking of the selection screen

Hello, on the selection-screen I have 4 date intervals. The are radiobuttons for each date intervals. The date field should be obligatory, but only if I select the radiobutton belonging to it on the selection-screen. How can I do that?

Radiobuttons are: r1, r2, r3, r4

intervals, are: s_date1, s_date2, s_date3, s_date4