Sneezing, is a normal reflex that occurs when there is something that is irritating the lining of the nose. Many dogs will sneeze a lot when around an environmental irritant like room deoderizing spray, perfume, or a lot of dust.
Unless you recently changed something like this in his environment, then I think this is an unlikely cause of his problem.
Sneezing can also be a part of upper respiratory infection. However, dogs with respiratory infections will usually also have fever, malaise and nasal and/or ocular discharge.
One of the most common reasons for fits of sneezing several times per day, is a foreign body stuck inside of the nose. These foreign bodies can be things like leaves, twigs, dead bugs, pieces of toys, etc....If this is the case, this will not stop until the foreign body is removed.
Other causes of chronic sneezing are allergies, neoplasia (cancer inside of the nose) and parasites of the nose.
Since this has been going on for 4 days now, with no resolution in site, then I would take him into the vet to be examined. You will want to use a vet that has a piece of equipment called a rhinoscope. This can allow better visualization inside of such a small nose, so that a foreign body can be found if present.
I am attaching a client information handout that discusses this problem in more detail. I hope that you find it useful.
I hope that this information is of help to you, and I wish you the best of luck with your dog. Please let me know if I can be of further assistance.