The first sentence of verse 4: لَّوْ أَرَ‌ادَ اللَّـهُ أَن يَتَّخِذَ وَلَدًا (Had Allah opted to have a son of His own) is a refutation of the allegation of those who said that angels were the progeny of Allah. The idea was false and absurd. Taking it on as a supposition of the impossible, it was said: If Allah Ta’ ala were to have, God forbid, any children, it goes without saying that it could not have happened without His intention and will, for it would have been impossible as children are not imposed on anyone - definitely not so in the case of Allah. Then, suppose He had the intention, in which case, everyone other than Him happens to have been created him, so He would have picked up one of them to be His progeny. Now, children have to be of the same genus as the father, and the created cannot be of the same genus as the creator, therefore, the intention to have progeny from the created becomes impossible.