Obito used Wood Release during his fight with Kirigakure Anbu because he was fused with Tobi/Guruguru, he served as a catalyst to make use of the Kekkei Genkai (the White Zetsu cells used to repair Obito's body are not enough to make this kind of Wood Release I believe, so it was a teamwork effort).
To further that point, Obito never used Wood Release in his battle with the Fourth Hokage... That's because he wasn't fused anymore with Tobi at this point. During the Fourth Shinobi World War, he still wasn't fused with Tobi, the guy doing shit on his own while fusing with the body of Yamato to strengthen his own Wood Release. That prove his use of the Kekkei Genkai isn't that stront since he's still a White Zetsu, even if special. We never see much of the White Zetsu army make use of their Wood Release on their own, only when fusing with each other (aka Naruto fighting an amalgam of White Zetsu).
So, why Obitio couldn't use it on its own? Because he can't, his Kiri Anbu fight thingy was marked with his symbiotic fusion with Tobi at the time!
