You have to use 4-7 cards of your elements mark, pillars/pends not included.
Using Submachine's example:
- you can use 2 Antlions and 3 Pulverizers (total of 5, so 4-7 is matched)
- use 2 Antlions, 1 Stone Skin, 1 Plate Armor, 3 Enchant Artifact and 1 Pulverizer (total of 8, so 4-7 is not matched)