Sigil of the Gatewatch

Endowed Planeswalker has "-2 loyalty: add a loyalty counter to each other Planeswalker you control."
Whenever a Planeswalker enters the battlefield, give endowed Planeswalker +1 loyalty
Endow {4} ({4}: Attach to target Planeswalker you control. Endow only as a sorcery. This card comes into play unattached and stays in play if the Planeswalker leaves play)