I can't really tell by those photos you posted, but if the engine has the smaller plastic tank, and the unit looks to have the 4-inch rims on it, I would assume that would be a 5HP Tecumseh. (edit .. . I see you posted its a 4HP, although I never saw a 4HP on a 2 stage snowblower, it could be)
If you can't find the engine model no. on the shroud, then just match up photos with one on Amazon, should be in the 5-7HP range for Tecumseh.