Gridview with Textboxes

I have a gridview with 4 textboxes in a row.User enters text in first Textbox in first row. Other textboxes are read-only and have their value auto-flilled based on value of first Textbox. When I press 'enter' or 'tab' key, textbox focus should go to first textbox of second row. How can I achieve this?.Tab index doesnt seem to be working. I'm doing it in VB.Net