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You can manage & track your card collection and decks once you create an account and login.

What happens if I use the tap ability and have fewer than 4 cards in my deck?

In that case, scrap all the cards in their deck. Then, setting those cards aside, shuffle all the other cards in your scrap pile into your deck. Then scrap enough cards from your reformed deck to scrap a total of 4 cards. The cards that arent Upgrades will be only cards in your scrap pile when youre done.
Source: Wizards of the Coast (2019-11-08)    【Link to Ruling】
Tags: Private Sidetrack, Battle Patrol • Demolitions