The 4th item can be None or an iterator yielding list items, which are
used to append() or extend() the object.  The 5th item can be None or
an iterator yielding a dict's (key, value) pairs, which are stuffed
into the object using __setitem__.

Also (as a separate, though related, feature) add "batching" for list
and dict items.  If you pickled a dict or list with a million items in
the past, it would push a million items onto the stack.  It now pushes
only 1000 items at a time on the stack, using repeated APPENDS or
SETITEMS opcodes.  (For lists, I hope that using many short extend()
calls doesn't exhibit quadratic behavior.)