Is there an adapter for an iMac thunderbolt 4 to a usb a ?

A Thunderbolt 4 port is the same as a USB-C port. There are plenty of USB-C to USB-A adapters. However, depends on what USB-A device you're trying to plug into a Thunderbolt 4 port.

Yes.

There is the branded > USB-C to USB Adapter - Apple

For other examples:

there are > https://www.amazon.com/dp/B077FY7831/ref=sspa_dk_detail_1?

and > https://www.target.com/p/philips-usb-c-to-3-0-usb-a-female-adapter/-/A-79804275