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Let's say, in theory, Lockdown has a 4 cards in hand, along with a Weapon, Armor, Utility, and one card underneath him. Lockdown flips to his alt mode, allowing him to draw 4 discard 4, but I reveal Hijack. Would I get to draw 4 while Lockdown's player has to pay the discard part of the effect or since he drew 0 cards he discards 0 cards?

You draw the 4 cards. The Lockdown player would have to scrap 4 cards. That poor, sweet fool has been Hijacked.
Source: Wizards of the Coast (2019-11-22)    【Link to Ruling】
Tags: Lockdown, Bounty Hunter