Answer from cs61c-bc (Svetlana Simakova 16608048) for Question 4
Can be reclaimed in both methods.

Ref.Count: since nothing points to upper left, it is deleted, which makes upper right and lower left 'not pointed to', and they are deleted, then, lower right also is deleted for same reasons

Mark&Sweep: upper left is not accessible from the root, thus no other can be marked, and therefore all 4 can be swept