Test four grade changes.

It has been brought to my attention that test 4 contains questions about VLANs,
which weren't covered in the early editions of Kurose and Ross and something we
didn't cover in class.  Because asking a question about VLANs under those
conditions is unfair, I've removed the questions from the test and adjusted the
test grades:

  Graduate - The grade for question 2 is 8; replace your previous grade for
  question 2 by 8 and recalculate your test grade.  If your previous grade for
  question 2 was 8, there is no change.

  Undergraduate - The grade for question 1 has been increased by two points up
  to a maximum of 8;  add two to your grade for question 1 up to a maximum of 8
  and recalculate your test grade.

Apologies for the inconvenience.  Let me know if you have any questions.