The Marines and the Warlords combined equal the four Emperors as stated by Garp.
There were not 5 Yonko level fighters on the Marines side because Sengoku was not Yonko level due to old age, Mihawk was though meaning it's 4V4
Garp/Sengoku/Warlords/a few top VAs cancel out the 1st,2nd,3rd commanders.
Rest of VAs cancel out the TR/Veteran lvl fighters
There were not 5 Yonko level fighters on the Marines side because Sengoku was not Yonko level due to old age, Mihawk was though meaning it's 4V4
Garp/Sengoku/Warlords/a few top VAs cancel out the 1st,2nd,3rd commanders.
Rest of VAs cancel out the TR/Veteran lvl fighters
So prove it. Who was the emperor before Shanks. Its pretty telling no such person was named when all the high bounties were revealed in 957.