So he completed and filed the Acknowledgement of Service form (so cannot argue non receipt of the claim form) and failed to file a Defence in time enabling you , presumably, to enter judgment in default of the Defence. Four months is too long but do you mean he waited four months following the default judgment or four months since he was served with the Claim Form? (or maybe since he filed the Acknowldgement of Service form). If from judgment then he probably has no chance. But if from an earlier date then what is crucial is the time since he was served with the judgment. He may have some explanation (e.g. emergency hospitalisation/home burnt down etc) but, as Anthony says, the judge will want to be satisfied that his proposed defence also has some merit.
You will not upset the judge by setting out your grounds for objection no matter how obvious they may already be to him.