Some dogs will only go in their own garden. As he's a puppy he doesn't realise that he can go on walks. It's confusing for him. Try to get him to go before you set off and take him into the garden as soon as you get home.
As he's only 4 months 40 minutes is too long to have him out walking. His bones need to develop and long walks can be detrimental. A walk of 15-20 minutes is enough for now.
Carol



