Answer from cs61c-el (David Tung 16479725) for Question 4
It can be reclaimed using reference.  The top left structure has no references to it, so it is reclaimed.  That leads the top right and bottom left structures to be reclaimed because they no longer have any references pointing to them, which then caues the bottom right structure to be reclaimed.

It can be reclaimed using the mark and sweep method if none of the objects in the picture are roots (can be accessed by the programmer).  In that case, they will not be marked, and thus they will be removed during the sweep phase.