Multiple checkbox. Correct/incorrect

$('.next21').click(function(){

       if (($('.twoone :checked[data-correct]').length != 4)&&($('.twoone :checked[value="incorrect"]').length != 0)){
                instp.open();
                $('input:checkbox').removeAttr('checked');
            }
       else if ($('.twoone :checked[data-correct]').length == 4) {
                $(".twoone").slideUp();
                $(".twotwo").slideDown();
            }     

    });

Does not work, this part of the code: ($('.twoone :checked[data-correct]').length != 4)

What to do?(