The entering creature must have power 4 or greater as it enters the battlefield, or Territorial Boar’s ability won’t trigger. Static abilities that raise (or lower) a creature’s power are taken into account. However, you can’t have a creature with power 3 or less enter the battlefield, raise its power with a spell, an activated ability, or a triggered ability, and have Territorial Boar’s ability trigger.

If the entering creature’s power changes to 3 or less after it has entered the battlefield, Territorial Boar still gets +1/+1 and gains vigilance.

If Territorial Boar’s power is raised to 4 or greater as it enters the battlefield, it will cause its own ability to trigger.