Don't know much about the 4th Kazekage but Oonoki and Gaara make a great combo and have been fighting together. You made this Oonoki prime which would mean he has more chakra. Gaara can sense Mu's presence with his sensor sand while Oonoki would blast him with jinton. Mu can counter or split to avoid a fatal hit but then Gaara would help Oonoki fight Mu. Using the splitting tech on Mu's behalf screws him over because he can't use jinton anymore.
The 4th Kazekage's gold dust is much slower than Gaara's sand. It can be made lighter by Mu though but the inverse is true too. Gaara can also control his father's gold dust but his father can't control his son's sand. For this reason, the 4th Kazekage is useless in this battle. Mu is definitely the strongest here but he would lose to prime Oonoki and Gaara with mid difficulty.
All in all Oonoki and Gaara win with mid-high difficulty.