[JS 输入框4个字符一分割] https://jsbin.com/koleleh/edit?html,js,output

character.js

function character(s){
    var memt = 16;
    s=s.replace(/\-/g,'');
    var len=s.length;
    var z=Math.ceil(len/4);
    var zz=parseInt(memt)+parseInt(z)-1;
    document.getElementById("sn").setAttribute("maxlength",zz);
    var ss='';
    for (var i = 0; i < z; i++) {
        ss+=s.substr(4*i,4);
        if(i!=(z-1)){ss+='-';}
        
    };

    return ss;
}