Hey Alan, consider though, have you ever personally needed 1/4000 or faster, do you really need that? Of the photos you've posted the last few months (all very lovely) I've not seen any that would need such a fast shutter speed. Unless you also shoot birds in flight?
You'd get used to having no front dial in a matter of minutes. My Canon 5D2 of course has front dial and within a few minutes of using my a6600 when it first arrived I didn't miss the dial at all.
I would say buy the a7c and try it out, you know you want to.

And if you don't care for it, sell it.