4.

Show that if e.VT < e'.VT then e happens before e'.

Equivalent to showing that if e does not happen before e' then e.VT !< e'.VT,
where !< stands for 'is not smaller than'. ( a -> b is equivalent to !b -> !a)

Suppose e occurs at process Pi and b occurs at Pj.

e.VT[i] = x

e'.VT[i] < x since there is no chain of messages from Pi to Pj at Pi's x-th
step or later (because e does not happen before e'). 

Therefore e.VT !< e'.VT.

Reference:
  http://faculty.cs.tamu.edu/welch/teaching/668.f02/install2.ps