Just a few notes about the second answer above.

The autotab in the example would happen after the fifth character when Len(text1.Text) = 5 (the first value greater than 4).

You, can still tab out with less than five characters, of course.

But it also remains possible to get more than five characters into text1 either by setting the value of text1 in code (e.g. text1.text = "123456789") or by pasting from the clipboard. Thus you can use length limits for your automatic "tab" when typing without absolutely precluding insertion of a longer string by other means.

Also the length limit can be represented by a variable that can be changed based on values entered in earlier textboxes or any other condition you wish to set.