Custom JavaScript verification is correct. Try using this:
// Parameters
var exclusiveColumn = 4;
var errorMessage = "Multiple items cannot be selected for the last column.";
// Run
var found = false;
$('input[name="hid_row_list_[% QuestionName() %]"]').val().split(',').forEach(function(row){
if (SSI_GetValue('[% QuestionName() %]_r' + row) == exclusiveColumn) {
if (found) {
strErrorMessage = errorMessage;
}
found = true;
}
});