<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>