I'll tell you what, you're as rich as anybody in the commodity that is required in order to pull it off...
Time, and you've got as much as anybody else. Just get some good advice from anyone you know, training material in the form of videos or books, and/or join forces with a local retriever club. Being four years old the first thing you should do is determine how strong his desire to retrieve is. Take him to to a field and throw some simple retrieves for him, is he stoked? Take to to some water and do the same, what is his attitude? Can he not get enough or does he lose interest? Get some feathers in the form of a dead duck or pigeon. What is his first opinion of one?
If his desire is strong, then it is up to you and your training skill & knowledge as to how far he can progress by the time season starts. Start with obedience and keep driving that home until he is rock-solid. AFTER that move on to some retriever training. Get him steady, help him improve his marking skills. That would be the meat & potatoes before season. If possible, you could move on to some more advanced retriever training but you probably won't master any others before season.
If his desire is NOT strong, strap on your thinking cap and think long and hard before you embark on the journey. Just because a dog is from a retriever breed doesn't mean they were born to be a hunter. Your average Golden is probably many generations removed from hunting ancestors. Just like Labs, they make awesome pets and that is primarily what they are bred for. The potential is usually there, but the chances that you can tap into it are not as high as they would be if the dog were from hunting parents.
Basically starting a four year old is just like starting a puppy. The up side is that you can assess up front what the likely outcome is going to be. Another up side is that they might already be odedient. The down side is that they WILL have some bad habits that are going to be hard, VERY HARD to break.
It takes a special dog to want to be a duck dog, and it takes an even more special trainer to do what you are wanting done. Doesn't mean YOU can't do it, just means you really have to work hard at it.
Just my opinion...
"I'll start spending less time with my dog and more with my wife when she starts fetching ducks for me"