You mean there are other choices? 😁.

Use of #4 solid has been mandated by local standards since the early Seventies— at least in Chicago and Reno. Even here (Memphis, Arkansas, and the Missouri boot heel) the use is routine. It was one of the few things about my service change that didn’t surprise the local suppliers.

Even though the NEC is quite clear that a GEC need not be larger than #6, and is silent regarding solid or stranded, local rules are common. Let’s review how we got here.
The first homes to have any manner of grounding often had a small (#10) wire connected to a water pipe. By the early Sixties the addition of a ground rod had become common, still using a small wire.
As Americans moved to the suburbs, these wires came into conflict with lawnmowers. This led to locales asking for ever larger wires, until #4 solid was accepted as “tough enough.”
Then came the weed hacker — and, by the late Seventies, a sharp rise in copper prices. Various surveys and inspections revealed the ground wires were either damaged or missing. Thus, protecting the wire with conduit became the norm. Living outside Chicago at the time, PVC was never an option.