Hi,
as far as I remember, yes.
But even if you could mix speed, you shouldn't do.
Your VSF links would become a bottleneck in your network.
From design perspective I always would use 10 Gig (or above) for VSF-links, where end devices wil communcate at 1 gig rates.
If budget is an issue, you could use just one connection between the switches. If you close the ring, you are still redundant. But I have to say,
that I would prefere to have two links between the switches.
From your ??? above:
Yes, you should close the ring and connect switch 4 back to switch 1.
Kind regards, Jörg