From experience, 4 weeks is by no means long enough for someone to be in top shape. Sure, go se a doctor, but 4 months is more likely to get your father back to his old self (actually better).
If your Dad has had a somewhat sudden change, as you describe, you definitely should contact his doctor, I think......it's probably something that can be treated.........it's possible that he was just trying to do too much, but better safe and call doc....