I think your best option is to have 4 textboxes with colons in between them, and then set the textboxes to limit to 2 characters and move to the next control when the 2 character limit is reached.
On top of that, you can add a hidden 5th field whose default value formula is something like: concat(firstPart, ':', secondPart, ':', thirdPart, ':', fourthPart)