But why? Why isn't fighting and defeating a YC2 enough for you?
However, Yamato remains as a strong candidate to sideline Sanji to number four, since the power level difference between Oden and Gaban is a big mystery. There are three options:
A) Oden was significantly stronger than Gaban and became number three of Roger, meaning that Yamato > Sanji will always hold.
B) Oden and Gaban were equals and this will apply to Yamato/Sanji.
C) Gaban remained number three of Roger Pirates and was slightly stronger than Oden at the end of their journey, meaning that Sanji will also surprass Yamato at some point during the series.