It seems there is a problem during the act change that does not show up until you try to load a save...
One person had a corrupt item in their inventory (a gem that was also a container) which caused a crash during the act change. The game goes through your inventory then; BF keys directly in your inventory are remove, the teleporter stones are removed entering act 4, and the names of set and unique equipment are changed (though the stats remain the same).
Try going through your inventory at the end of act 3 and see if you can find anything that appears corrupt. The spear / rings that can be found in the BF shouldn't cause a problem, but that is the kind of thing to look for. Drop any leftover quest items, BF keys and the teleporter stones, and sell any spare equipment you do not need. After that, try entering act 4 and see if you can save the game and load.
