Answer from cs61c-cl (Hiroki Terashima 16032281) for Question 4
Reference count method- yes
mark and sweep method - no

Under reference count method, the four boxes will have 0(top-left), 1 (top-right), 1(bottom-left) and 2(bottom-right) as the number of pointers that point to them.  So the top-left will be deleted first, along with its pointers, so after one phase, top-left will be gone, top-right and bottom-left will have 0 pointers pointed to them, and bottom-right will have 2.  Doing this again will successfully reclaim all the boxes.
Mark and sweep method won't work because there is no way to access the top-left box since there is no pointer that points to it.