Defeating two sweet commanders gave Luffy a higher bounty than Marco. Even though Marco was with the WSM for 30 years and tussled wit admirals.
Defeating two commanders +
invanding a Yonko territory + destroying her Tea Party, which was a worldwide known important event for said Yonko + commanded both Firetank Pirates, Germa 66 and the Sun Pirates (a little bit of fake news, but it got added to the feats anyway).
Yonkos do not get 4B bounties based on their own strenght alone.
Their armies, their deeds accross the years, their influences, etc, ALL OF THESE count. Commanders are mere units from a Yonko Army. Enough to get them to the 1B+. But most likely not enough to reach 2B.
And just to bring some reality check to some people, Mihawk is a lone wolf. He has no armies, no allies, no vast territories. He has a single castle somewhere and a casket that he calls a boat. Dude couldn't deal with a freaking solo WB commander. It would take some really bad shrooms for Oda to give this former Shichibukai a bounty higher than 2B. And I say 2B being generous because he's Zoro's endgame opponent.
YC commanders like Katakuri and King should not be given bounties near 2B because they are soldiers, not leaders themselves.
Marco is a different story currently, due to the fact that he was, for a while, leader of the WB pirates remnants. But they soon lost to BB, so I don't even think he got a lot from it.
Bounties, despite not being straight power levels, they are, in fact, power level indicators. But there are ceillings that non-leaders should not break, because it makes ZERO sense.