The cost saving is more than $4, IMHO.
Espressif has a public available forum and constantly updating SDK for their chips, that means you can run your own C code in it, and get the most performance out of it. Also, the 80MHz CPU can be abused up to 160MHz without any problems at commercial temperature, so you can further push it to higher performance.
Unlike a CC, it can be expanded with only a GPIO expander to make a true SoC solution, no external MCU or CPU needed, so this is another $3 down.
Finally, this chip has available hardware design that is not FCC certified, but FCC verified, so you can simply copy the design and you know it will pass the certification.