required validation - only need 1 out of 4 textboxes

hi all,
i have a search form, the form has 4 textboxes
and there's a search button,
when i click the button, i need it to check for input in EITHER of the 4 textboxes
so if there is input in any of the textboxes it would do a search, but if the 4 textboxes are empty, i want a message saying a search criteria is required

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