This is what I did in Version 7.1.2, but it should work in 7.0.
I created four TextFields: TextField1, TextField2, TextField3, TextField4. If it is a TextField, under the Field tab, I also limited the max. characters to 1. If it is a numeric field, you don't have that option, but using the script below will limit the maxLength to 1 character.
var maxLength = 1;
var currentLength = xfa.event.newText.length;
// Change focus to next field if maxium length reached.
if( currentLength == maxLength )
What this script should do, is once it reaches the maxLength (being 1), it sets the focus to the next field. Add this script to the second field, changing the object name to "TextField3", etc.