so...max 4 shards.
4 in total,,or from each element?
If you use four shards, you are only allowed one copy of every different card which you use (aside from elemental pillars and stuff) So even your four shard would have to be in different elements, because you wouldn't be allowed more than one copy of each different card (and shard is a card as well)
So to build a reasonable deck, two shards is probably the relative maximum. (Since that would still allow you to play three of each different card in your deck.
--> At least that is my interpretation.