I’m using World Clock. Has a Big Sur widget and companions apps on iOS, showing 4 zones in a small widget. One-time purchase and does what it says on the tin very well!
If you’re on Big Sur (and iOS 14) you have a world clock with four time zones included in the widgets Apple provides. No cost – you already own the widgets.
You can have a “medium” or a “small” version – both at the same time – so that is two clusters with four zones in each.
I can’t recommend The Clock by Seense highly enough. Available for Mac, iOS, iPadOS. Syncs across all platforms, so adding a time zone in one app will sync to all. Has a slider so you can change a time in a zone. Pomodoro timer available. Great app. Not free but very reasonable. I would not expect to get an app of this quality for nothing. Developers have to put food on the table, too.
Thanks everyone for the excellent recommendations. I purchased The Clock by Seense–it is exactly what I was looking for. Thanks again for the kind assistance!