The HKs are well above a Yonko crew.
Firstly, most Yonko crews can't deal with regen, and even then it'd usually only be the Yonko who could do so. So everyone else automatically loses. No evidence of Marco/Katakuri ever being able to deal with regen - King would be alright since the HKs wouldn't be able to damage him either.
Secondly, Yonko crews usually have a top-tier, 3-4 main fighters then a bunch of veterans. The HKs have Shamrock the top-tier, then so far, Gunko, Super Sommers and Killingham seem to be around the upper end of high-tier, say YC1 or even YC2 at worst. That already puts their top fighters above a Yonko crew's, which has only one YC1 and one YC2.
Furthermore, I don't see why Imu would have veteran level fodder amongst their HKs, especially when they're tryna recruit guys like Loki so clearly they value strength. I'd be surprised if all of the HKs aren't individually high-tiers, and even if they aren't, no one's beating them anyways because no one bar the top-tier in Yonko crews would know how to overcome regen (post-Elbaf SH crew aside ofc).
Of course, the difference between top-tiers and everyone else is so huge that if the top-tier is strong enough, they could carry a victory. For example, Kaido could probably solo the entire HKs bar Shamrock. So if everyone else could hold their own until Kaido beat Shamrock then the Beast Pirates could win. But overall, the HKs are much stronger than your average Yonko crew.