A.T. Robertson Commentary Acts 4:27

"for of a truth in this city against thy holy Servant Jesus, whom thou didst anoint, both Herod and Pontius Pilate, with the Gentiles and the peoples of Israel, were gathered together," — Acts 4:27 (ASV)

Both Herod and Pontios Pilate (Hηρωιδης τε κα Ποντιυς Πειλατος). Luke alone (Luke 23:12) tells of the reconciliation between Herod and Pilate at the trial of Jesus. So Peter and the rest interpret this prophecy as directly fulfilled in their conduct towards Jesus Christ.

Whom thou didst anoint (ον εχρισας). As in verse 26 (cf. Luke 4:18; Isaiah 61:1). Inaugurated as King Messiah.