<form>
  <input name="card_no1" type="text" size="5"  maxlength="4" value=""  onKeyup="if(this.form.card_no1.value.length>=4) this.form.card_no2.focus();" />
  -
  <input name="card_no2" type="text"  size=5  maxlength=4 value="" onKeyup="if(this.form.card_no2.value.length>=4) this.form.card_no3.focus();" />
  -
  <input name="card_no3" type="text" size=5  maxlength=4 value="" onKeyup="if(this.form.card_no3.value.length>=4) this.form.card_no4.focus();"  />
  -
  <input name="card_no4" type="text" size=5  maxlength=4 value="" onKeyup="if(this.form.card_no4.value.length>=4) ;" />
 </form>