It's like if Emperor Palpatine killed Darth Vader in the opening scene of "Star Wars Episode VI: Return of the Jedi"...
The man himself even provided his own illustrations for this creature, clearly outlining the four-legs feature."
Do a google search and you'll find at least a couple of Tolkien's original drawings of Smaug with four legs.
The two legged version of Smaug from the movie is pretty cool, but I literally painted my illustration a few years before it even existed.
People do draw Smaug in said- style, so go and enjoy it instead of moaning.