You have overexposed by 2 stops which is not a huge disaster depending on subject matter. Most people will rate their film down by one stop to ensure good shadow exposure. 2 stops might lose some highlight detail.(posted 8098 days ago)If the roll has important images, i would expose another roll of 400 rated at 100asa of the same or similar subject matter in similar lighting conditions and then clip it in fourths and develop each one with a one minute differential starting at 10 and ending at 7. Then make some contact prints of each.
This may sound a little tedious, but it will allow you to better understand the relationship of developing time to neg density. You may discover you like the look of your "mistake". Also, if you are serious about photography beyond the class, you will be doing ths same type of thing (called testing) with all film and developers you use to learn how to get negatives that suit your style.
Finally, if you are a student learning about photography, there are no stupid mistakes. Quite a few photographers would agree that making mistakes (or breaking the "rules" on purpose) is actually one of the most efficent ways to learn the craft.