I've read on several pages that, a website hosted on a IPv6 address will not be accessible from any PC with IPv4. Is it so? And what about accessing a website hosted on IPv4 from a PC with IPv6 assigned??

I've read on several pages that, a website hosted on a IPv6 address will not be accessible from any PC with IPv4. Is it so?

Correct. If the website is IPv6 only and the PC is IPv4 only, then they cannot communicate directly. IPv4 and IPv6 run independently of each other.

And what about accessing a website hosted on IPv4 from a PC with IPv6 assigned??

Same thing.

With that said ... It is rare to have a website reachable via only IPv6. Most are dual-stack, meaning that they have both IPv4 and IPv6 addresses. Or, the DNS records return addresses of both IPv4 and IPv6 servers.

forum.mikrotik.com is reachable via IPv4 or IPv6. Unless you are specifically looking, you wouldn't know which address you are actually connecting to.

Likewise, it would be unusual to have a PC running IPv6 only. It would most likely be dual-stack.