The book in the Bible that immediately follows the four Gospels is Acts, also called the Acts of the Apostles.
Acts begins with Jesus’ ascension and follows the growth of the early Christian movement. It opens in Jerusalem, where the apostles receive the Holy Spirit at Pentecost, then traces the movement’s expansion through Judea, Samaria and the wider Roman world. Peter dominates the early chapters, while Paul becomes the central figure later.
Acts is traditionally attributed to Luke and is commonly understood as the second volume of Luke’s Gospel. The two books are addressed to Theophilus and share vocabulary, style and narrative themes. Their separation in the New Testament reflects the convention of grouping all four Gospels together.
A common mix-up is confusing Acts with Romans, the first Pauline letter. Romans follows Acts, but Acts is the fifth book of the New Testament and the canonical bridge between the Gospel narratives and the letters.