"ASCII layout of one of the 2 possible 4th (last) levels of Sokoban...

This shows an intial series of moves that allow the puzzle to be solved without "freezing" any boulders.
[Other solutions could well be possible]

None of the levels in Sokoban require 'freezing' a boulder; all extra boulders are "spares" in case you break or lose one (or more)."