This situation came up in a tournament the other day and I've heard 2 different ways of interpreting it. My friend is playing ichorid and has 2 dredgers in his graveyard. He plays breakthrough with his only dredgers in the grave being a golgari-grave troll and a golgari thug. He dredges the troll and thug which places another dredger in the grave. His opponent says that he cannot dredge the third dredger because you have to have the 4 legal dredge targets in the graveyard when breakthrough resolves in order to dredge all 4 breakthrough draws.