If one were weaker than the other, they would have been defeated and would have lost their position as one of the emperors. The Yonko may command different levels of influence, may control lesser or more land than others, but one thing that the fandom has come to a consensus is that the Yonko are equally matched in terms of strength and power.
Just like how the Supernovas of the Worst Generation are rivals and are more or less equally matched, the Yonko are at a draw with each other - and a war between two Yonko's would lead to a major loss on both sides and the possibility of both captains loosing the 'title' of Yonko.
We also know that Kaido had set out to attack Whitebeard while the latter was on his way to Marineford, proving that Kadio wasn't afraid to pick a war with Whitebeard and was confident he could manage defeating Whitebeard and all his allies. Shanks appeared and stopped Kaido from attacking Whitebeard, Kaido backed out and while we can only speculate why, the most likely answer is that Kadio didn't want to wage war with two fellow Yonko's at the same time.
There are some who think that Kaido backed out due to the fear that he would suffer defeat against Shanks, while this is also a valid opinion it is very unlikely. Would you agree if I said that Kaido knew 100% that he would achieve victory against Whitebeard and set out to attack him? No. Would you agree if I said that Whitebeard didn't face Kaido himself because Whitebeard knew he was the weaker one of the two? No. For the same reasons, Kaido backing away from a fight against Shanks isn't a sign of weakness and shouldn't be seen as such. Whitebeard had to reach Marineford at the earliest, and didn't face Kaido himself. Kaido decided not to confront Shanks due to the possibility of waging war against two Yonko's at the same time - and possibly no benefit coming out of fighting Shanks while his actual target was Whitebeard.
We are also aware that the Yonko continue to rise in strength and power, Kaido with SMILE's and Blackbeard with extracting devil fruits out of its original users. Although we haven't seen how Big Mom is increasing in strength or power, she did deal with Caesar in the past and we can be pretty sure that she and her crew isn't sitting idle. Shanks is more laid back, but at the same time we certainly cannot rule out that he isn't participating in the power play, he has to keep up with the other four to not be at a disadvantage.
If we were to compare this scenario with our world, it would be the period of Cold War between USSR and USA. If one had a certain technology, the other had to come up with something similar to stay in the race. They had to struggle over influence in various regions, and continued to try and match each other in terms of development, influence and arms.