Scrapyard Enforcer

Forge {4}{u}{b} (You may cast this spell by paying its
forge cost and discarding an artifact card.)
When Scrapyard Enforcer enters the battlefield,
if it was forged, mill four cards. (To mill a card,
put the top card of your library into your graveyard.)
At the beginning of your end step, draw a card
for each artifact card in your graveyard.