Yea akainu is maybe but not in general. Marco held his own against admirals it's a fight of attrition since no one can do damage to each other on a high level. Kizaru couldn't injure him at all and Marco barely kicked him so I wouldn't just randomly say yonko FM<admiral. Also Ben beckham was given quite a bit of hype with kizaru. Jack might be different based on the other calamities. But Marco and Ben seem admiral level meaning they can take on the likes of Fujitora.
Personally neither one of these characters are admiral level imo, I have argued against Marco multiple times but Beckman idk. Given the fact that Shanks crew is much smaller I see the core crew members stronger than WBs core crew members, Shanks-quality Whitebeard-quantity.
Also its important to remember what Marcos fruit does, it allows him to regenerate from any type of damage no matter who inflicts it. Its perfect for stalling of wearing out your opponents which made him the perfect opponent for the Admirals. He slows them down in order for everyone else to get to Ace.
Based off what I've seen offensively from this fruit its good but not great, Marco kicked Kizaru and did nothing, he couldn't even tickle Akainu without Vistas help indicating Marcos offensive power is not Admiral level.
Ive argued this before and people tried to act like I was bashing Marco but that's not the truth, I have analyzed his abilities and came to the conclusion that the only thing that makes people think he's admiral level is because hes WBs #1 but nothing Marco done indicates that FMs are suppose to be admiral level. He also had no reason to hold back during the war as both Ace and Whitebeard died, if he had the power to defeat the admirals and he didn't its partially his fault for their deaths.