Grant Thornton, BDO, and RSM are the next step down. There are a few other international firms and then there are a large number of international alliances.

The second tier firms operate very similarly to the Big 4 but are an order of magnitude smaller. They'll do audit work for public companies, just not very big ones. Or they'll do pieces of work for bigger firms. But companies can still be public with revenues of only a few hundred million and they'll be far more price sensitive than Google or Goldman.

Just like in cloud computing or AI scale has serious advantages for accounting firms and it is difficult and expensive to try to make the jump to the top tier. Better to stay in your target market in terms of client size, geography, and what price you charge, than to try to drastically change your business. Far too likely to lose your existing business and not succeed in the new one.