15 cards where the number of colors is 4

A-Omnath, Locus of Creation (Zendikar Rising #A-232)
Aragorn, the Uniter (The Lord of the Rings: Tales of Middle-earth #192)
Atraxa, Grand Unifier (Phyrexia: All Will Be One #196)
Atraxa, Praetors' Voice (Double Masters #190)
Breya, Etherium Shaper (Double Masters #192)
Dune-Brood Nephilim (Guildpact #110)
Glint-Eye Nephilim (Dominaria United Commander #152)
Ink-Treader Nephilim (Guildpact #117)
Kynaios and Tiro of Meletis (Commander 2016 #36)
Omnath, Locus of Creation (Zendikar Rising #232)
Saskia the Unyielding (Commander 2016 #41)
The Fourteenth Doctor (Secret Lair Drop #1583)
Witch-Maw Nephilim (Guildpact #138)
Yidris, Maelstrom Wielder (Commander 2016 #50)
Yore-Tiller Nephilim (Guildpact #140)

The message was clear to Sorin—time to search for something else.