Stampede Rider

• WWizards of the Coast2020-01-24
  Official Ruling
  Once Stampede Rider’s ability has given it +1/+1, it won’t lose that bonus if you no longer control a creature with power 4 or greater later in the turn.
• WWizards of the Coast2020-01-24
  Official Ruling
  If you don’t control a creature with power 4 or greater as your combat phase begins, Stampede Rider’s ability doesn’t trigger. If you don’t control one as the ability resolves, it has no effect. It doesn’t have to be the same creature at both times, however.
• WWizards of the Coast2020-01-24
  Official Ruling
  Stampede Rider gets only +1/+1 from its ability, no matter how many creatures with power 4 or greater you control beyond the first.

Rulings via Scryfall