I haven’t used the C yet, I’ve only used the 4 - I’ve a “For Mac” Bluetooth one which I returned because the 2.4GHz wireless version was performing much better. I suspect they made the Bluetooth one to sell in the Apple Store. As far as I know, they haven’t changed the basic mouse design in 5-6 years now. It still works and feels high quality, I’ve no complaints. (Except that it is definitely not “travel size” - and isn’t rechargeable, it takes an AA battery - but for all that, it has a nice high quality plastic feel and the design of the mouse means you’re not rubbing your palm all over the table as when trying to hold a regular mouse with a vertical wrist.)

The extra buttons require their driver but it works fine without and I don’t notice the buttons most of the time anyway.

I had every VerticalMouse so far (they don't seem to last for me for whatever reason). I prefer the C, it sits nicer in my hand, they changed the shape; I can't comment on the buttons as I under-utilize those. Either way though, going to any VerticalMouse from a regular mouse is a huge improvement. The changes between 4 and C are probably hard to pick out for someone who hasn't exclusively used those mice for a decade.