My iMac is early 2009 with 4 GB memory and 213 GB free storage. Is there any way to upgrade to High Sierra?

There is a way but it is unsupported, not all features will work, wi-fi could be an issue. It's generally not the best idea.

Assume you won't get any help when things go wrong and don't work.

That being said, you can do a search for High Sierra for unsupported Macs.

Not something I would do.

Nope.

That machine is too old.

The early 2009 iMac is maxed at running El Capitan. It wasn't

compatible with Sierra, let alone High Sierra.