They literally can. Yonkos aint anything special
I don't think so man, at least not like this.
Let me tell you this.
Let's say that gb is sanji's opponent and fujitora is Zoro's. I will argue against you that fujitora>gb but i will not argue that fujitora would beat shanks or he will take him to extreme difficulty fight because oda show us something.
Shanks did something crazy but it's Luffy's next thing, either fight him or the guy that beats him.
It's crazy but after Luffy's next pu, i believe he could fight 2 admiral level opponents and this shouldn't be a problem for as.
It's always the same thing: kaido king queen
Crocodile mr1 mr2 .. Luffy vs whoever zoro and sanji vs admirals. Nothing will change in the power scale no matter who sanji and zoro will face..
But zoro and sanji will be yonko level characters in their PRIME.
Both will defeat admiral level opponents in their early twenties, imagine when they will hit their primes?( Usually 40 by oda's preferences)
Just remember one thing...
Zoro's opponent > sanji's opponent and sanji after his pu individually..