function focus_move(no){
 var len=document.getElementsByName('vip_num'+no)[0].value.length;
 if(len==4){
 try{
   document.getElementsByName('vip_num'+(no+1))[0].focus();
 } catch(e){}
 }
}
//숫자만 입력하게//
function ncheck(num){
  var chkflag=true;
  for(var i =0;i<num.length;i++){
   c=num.charAt(i);
      if(!(c>='0' && c<='9')) {
              chkflag=false;
              break;
      }
  }

  return chkflag;
}
function numchk(v,n){
 if(!ncheck(v)){
   alert('숫자만 입력하세요');
  var temp=eval("document.sform."+n);  //form이름이 form1이더군요.. 1추가했구요..

  temp.value='';
  temp.focus();
 }
 }

<td class="pd_l10 pd_t10 pd_b10 bd_tb bg_f5" width="510"><input name="vip_num1" type="text" class="s" size="10" onkeyup="numchk(this.value,this.name);focus_move(1)"  /> - <input name="vip_num2" type="text" class="s" size="10" onkeyup="numchk(this.value,this.name);focus_move(2)"  /> - <input name="vip_num3" type="text" class="s" size="10" onkeyup="numchk(this.value,this.name);focus_move(3)" /></td>