(Ignoring the "Canonicy")

Well, from what I understand about Saiyan mythology!
The "Super Saiyan" was a blend of a normal Saiyan and an Oozaru, a being with unimaginable power which eventually lead him to kill himself.

Now, When Goku ascended to the first level he transformed, but the only arguable different was his hair, which could easily be caused merely by the flow of Chi.

Also, when Vegeta saw Broly's "Legendary Super Saiyan" form, he maybe thought this because of Broly's Oozaru-like body structure, this would explain why Super Saiyan 2 is arguably stronger and why Trunks reached a VERY similar form to Broly (Ultimate Super Saiyan).

Super Saiyan 4 is caused by becoming a super-powered Oozaru known as a Golden Oozaru, this means that the Oozaru form has a very similar form to Super Saiyan, making me believe Goku's original Super Saiyan form was merely an extra-powered version of himself, same goes for all other forms.
Super Saiyan 4 is a blend of both Oozaru and Super Saiyan, making me believe that Super Saiyan 4 is the REAL Legendary Super Saiyan form.

Also, in the anime (though it's a filler scene) we see the original Super Saiyan depicted as what we call a 'Golden Ozaru' which is the form that we all know now leads to Super Saiyan 4.

What do you guys think?

I actually consider GT as "fake" because Akira Toriyama didn't make it.
but if you put it that way then yea, it sounds like Super Saiyan 4 is "The Legendary Super Saiyan" transformation. ^^

Yeah, that's why I said "Ignoring the canonicy" at the top of the post :P