Adam Clarke on Acts 21:4

Who said to Paul through the Spirit - We cannot understand this as a command from the Holy Spirit not to go up to Jerusalem, else Paul must have been highly criminal to have disobeyed it. Through the Spirit, must either refer to their own great earnestness to dissuade him from taking a journey which they plainly saw would be injurious to him - and so Bp. Pearce understands this place; or, if it refer to the Holy Spirit, it must mean that if he regarded his personal safety he must not, at this time, go up to Jerusalem. The Spirit foretold Paul's persecutions, but does not appear to have forbidden his journey; and Paul was persuaded that, in acting as he was about to do, whatever personal risk he ran, he should bring more glory to God, by going to Jerusalem, than by tarrying at Tyre or elsewhere. The purport of this Divine communication was, "If thou go up to Jerusalem the Jews will persecute thee; and thou wilt be imprisoned, etc." As he was apprized of this, he might have desisted, for the whole was conditional: Paul might or might not go to Jerusalem; if he did go, he would be persecuted, and be in danger of losing his life. The Holy Spirit neither commanded him to go, nor forbade him; the whole was conditional; and he was left to the free exercise of his own judgment and conscience. This was a similar case to that of David in Keilah, 1Sam 23:9-13. David prevented the threatened evil by leaving Keilah: Paul fell into it by going to Jerusalem.