In the Greek sentence structure verse 4 is in indirect discourse; direct discourse begins in verse 5 of the Greek text. The Good News Translation has made both verses into direct discourse (so also the New English Bible).

The passive expression is being kept a prisoner may be rendered in the active form by “soldiers are guarding Paul in Caesarea” or even as “Paul remains a prisoner in Caesarea.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on The Acts of the Apostles. (UBS Handbook Series). New York: UBS, 1972. For this and other handbooks for translators see here .