They zealously affect you, [but] not well; yea, they would exclude you, that ye might affect them.
Note 10 at Ga 4:17: The New International Version translates this verse as "Those people are zealous to win you over, but for no good. What they want is to alienate you (from us), so that you may be zealous for them." This seems to accurately represent what the King James English is trying to convey.
Paul had addressed the false teachers' gospel, and here, he was addressing their motives. He was stating that they were zealous but not in a good way or for a good purpose. Their intention was to alienate the Galatians from the true Gospel and from those who taught it, so that instead of the Galatians going to Christ, they would go to them (the Judaizers).