Javascript on keypress + sign, how do I retrieve it

Currently I got the following script for my enter key

```
<script type="text/javascript">
$(document).ready(function() {
$(function(){

    $("input[type=text]").keyup(function(e){

        if(e.keyCode == "13")
        {
            var nextIndex = $(this).prop('tabindex') + 1;
            var selectedId = $(this).prop('id');
            var rowSplit = selectedId.split("_");
            var rowIn = rowSplit[1];

            if(selectedId.indexOf("big") > -1)
            {

            bigFunction(rowIn)

            $('[tabindex=' + nextIndex + ']').focus();
            }

    $("form :input").on("keypress", function(e) {
    return e.keyCode != 13;
    });

});
</script>

```

I want it to work for if e.g 

I got a text field with maxlength 4

I enter 4 random number e.g 1234 and then I press the letter + sign, I want to change focus to next tabindex

**How do I do it, thanks for helping so much !!**