as far as i am concerned, its no shape shifting, someone else was under that mask before obito went under, thats all, it still seems impossible that obito could have fought and if he did put up such a good fight against the 4th while controlling kyuubi at the then young age he would have been at, not to mention that he was completely oblivious to the speed and some of the abilities of the 4th, which obito should not have been since the 4th was his teacher. My opinion is that madara rescued tobi from under the rocks and saved his life then made him his apprentice, because itachi did mention that it was madara that he met with to take out the uchihas, and if yall are gonna tell me that obito was smart enough to pull the wool over itachi's eyes after all the we saw itachi planed ahead for, then you people must be crazy. Also to be noted, madaras words during the kage fight when he was speaking to kabuto through muu, he said and i quote "DOES HE ALSO KNOW ABOUT OUR PLAN", THATS WHAT MADARA ASKED HIMSELF WHEN TALKING TO KABUTO, tell me if i am wrong, but there is no way , no explanation that can come about to make me believe that obito fought the 4th hokage while controlling kyuubi and put up such a good fight, absolutely non,