I think that their just normal SSJ, only their power is maxed to the fullest. The Golden Oozaru probably couldn't handle so much power because the form was so primintive. So what did it do? It compensated the excess power by making Goku and Vegita's body smaller, compacting the power in tiny bundles, sort of like when your body stores fat for later use. I have a perfect case scenario. Recently, I aquired episodes 57-60 of Dragonball GT. I know your asking "Whats the point?". This is the point.

During one of the episodes, Goku flat out got his ass beat by Li-Shenlon. As a result, he reverted back to his kidlike state and all of a sudden, Gohan comes along with Goten, Trunks, Videl, Chi-Chi, and Uubu. All of the fighters excluding Uubu surround him to give him a charge of Saiya-Jin power. Then something funny happens', he tells them to sort of "Jump start" him, which leads me back to my point, the stored up energy. I think Goku needed the stored up energy not just to go SSJ4, but also to maintain it.

Next, ever wonder why they never tried to go SSJ while in that form. Well, it all leads back to the stored energy. To go to the next SSJ, they would have to have the potential and strength to, which they did. But, yes there is a but, the release off all that stored energy would do one of two things, destroy the Earth or their bodies. As you also may notice, when Goku, Vegita, and Gogeta power up, their is a golden aura surrounding them. This more then proves my point. This mean that most likely than not, their SSJ and could go to the next stage but after about 10-20 seconds, they already look worn out and while fused, they can only stay fused for only 20 minutes.

Now, who is the strongest SSJ4? For this, the old "Brutze" Ray Bulma made will come in real handy for this. While fighting Li-Shenlon, Vegita comes on the scene telling that he will go SSJ4 to. The Brutze Ray made Vegita go normal Oozaru but not Golden, yet. As he was squeezing the hell out of Goku, he relized who he was, controlled it, and you guest it, went SSJ4. I doubt that Vegita is the strongest because as Goku's transform was without aid of any technology, Vegita's transform was made possible by technology, giving him some fault's without beign a natural process.