I can see enable utf8mb4, which I assume converts utf8 to utf8mb4??

How long will it take to convert a 5gb database?

It's nearly impossible to predict how long that will take, as it is dependent on the server hardware and its setup, but it could take several hours. If you have a beefy server and a well-tuned SQL engine, it could take much less time than that, a matter of minutes.

3 hours ago, Mark H said:

It's nearly impossible to predict how long that will take, as it is dependent on the server hardware and its setup, but it could take several hours. If you have a beefy server and a well-tuned SQL engine, it could take much less time than that, a matter of minutes.

It's a dedicated server.

I was concerned that it would time out if it has to run in the browser. Or does it run in the background like when it did all those background processes upon upgrading to IPS4?

If run from the ACP with v4, it should not time out as it's done in "batches", though I believe you do have to remain on that ACP page while it runs.