If he is four months old, then you should see if you can get your dog into a puppy class of some sort.
Although I've been to two obedience, but I've just joined Mornington Obedience and their Puppy Socialisation class is absolute superb! Its the best thing in the world for new pups as they spend time socialising and also spend time doing obedience.
All their socialisation is done off lead in a little pen area and the pups will either mingle with each other or just wander around doing their own thing.
If you can find an obedience club or a place that does this type of socialisation, then its great to join in.
When I attended Mornington today, she enjoyed doing obedience, but socialisation, she didn't seem to really want to join and play with the other pups, and she was only 12 Weeks old! (Just 3 months).
So 4 months is not too old, more too young of I may say so! (In a good way! lol). I know Southern Obedience do socialisation as well, but I am not sure if they do it off lead, but I didn't stay long enough to watch the entire thing, but I am intending to join theirs once Bella is finished with Puppy Socialisation and officially starts the Obedience work and I think she will be 4 months as she is very intelligent and adapt new skills easily.
But my advice from what I have learnt from my question on this forum last week and socialisation, is to do it at your own pace and let the dog do things at their own pace. Don't force them to do things if they tire or do not want to do it, and you will find they will become better companion if you understand them!
Hope this help!