My guess for what the hints meant (since we only got Yamato stuff so far)
>the "4" was referring to the number of prisoners, counting Yamato
>The Jack stuff is because he might appear as a child in the flashback
>The Kitetsu might be confirmation that Ushimaru used to wield the Nidai
>The scene from Alabasta might be Ushimaru using the breath of all things and thats a Shimotsuki hability