Hm. So maybe if Sichuan is not conquered by Qin, and Qin/Zhao and Yan dominate the north (maybe Yan conquers Korea), would that be a possible balance of powers?
Why exclude Sichuan? Without it the Qin are greatly diminished. A determined opponent could work them loose from the Yellow River, at which point they'd be too small a state to survive.
The key part of the scenario is each significant state having its own Sichuan - a source of taxes and recruits that the other states couldn't get at. That kind of security renders it almost impossible for another state to repeat the feat the Qin did in unifying China - rapid defeat and annexation of opponents one-by-one in a period of weakness.
Think of the last two really large states to survive: Qin and Chu.
The Chu lasted the longest because they were on the rivers, and - Wu and Yue aside - none of their opponents habitually fought war in that kind of environment. All of the river valleys south from the Yangtze plus Zhejiang represented a secure flank for the late Chu. A lot else to the north and west could be overrun without breaking the Chu as a player.
Or imagine if Qin suffered some big setback in 230 BC, just before they ran rampant over everything. Imagine their entire military is destroyed in the field. Even then, it's hard to imagine one of their rivals annexing the Qin state. Sichuan was inaccessible to the other states, the Wei valley nearly so, and the upper Yellow River a tough nut to crack. In the face of incredible disaster, they would retain a very strong chance of surviving. A power controlling the whole core of the Chinese plains could squash the Qin as in OTL the Qin did the Chu, but a divided field they could survive.
That is a model of the kind of state I could imagine surviving the Warring States period: One with some territory in the center, but enough territory behind geographic barriers to be secure, and
not facing a power that controls the Central Plains.
I think you could have three of those, perhaps four somehow (Qin, Zhao, Yan, and Chu? Zhao, Chu, Wu/Yue, and Yan?). But it's easier if someone annexes Sichuan. And given the inherent weaknesses of the south, it's probably more likely to happen if the south is unified between Wuhan and Zhejiang/Yue.