Mobilized District

• WWizards of the Coast2019-05-03
  Official Ruling
  If you control four or more legendary permanents that are creatures and/or planeswalkers, the last ability of Mobilized District costs no mana to activate. A legendary planeswalker that's also a creature reduces this cost by only .
• WWizards of the Coast2019-05-03
  Official Ruling
  If Mobilized District becomes a creature the same turn it enters the battlefield, you can't attack with it or activate its mana ability.
• WWizards of the Coast2019-05-03
  Official Ruling
  An ability that turns a land into a creature also sets that creature's power and toughness. If the land was already a creature (for example, if it was the target of Awakening of Vitu-Ghazi), this will overwrite the previous effect that set its power and toughness. Effects that modify its power or toughness will continue to apply no matter when they started to take effect. The same is true for counters that change its power or toughness (such as +1/+1 counters) and effects that switch its power and toughness. For example, if Mobilized District has been made a 0/0 creature with nine +1/+1 counters on it, activating its last ability will turn it into a 12/12 Citizen creature that's still a land.

Rulings via Scryfall