This isn't possible natively now, you can setup a cloudflare tunnel to proxy traffic from 6to4.

I have never used Cloudflare Tunnel, is there an additional cost? Can I keep communication private?

Yes. However, you will need to look up a tutorial on how to set it up. Native Ipv4 support is on the roadmap but not yet out.

I think it won't work in my case, I need questdb to be called with ipv4, but if I configure the tunnel it will still try to call questdb with ipv6, won't it?

Where are you seeing that QuestDB has no IPv6 support? The ODBC driver for QuestDB implies that IPv6 is supported under the IP Version option [0]

Using a connection string that points to the railway.internal domain hosting the QuestDB instance should work natively if your QuestDB instance is listening on [::] correctly.

[0] https://docs.devart.com/odbc/questdb/connection%5Foptions.htm

For ODBC i dont try, but in the official client this options doesnt exists

worked with this configuration:

http::addr=[questdb.railway.internal]:8812;username=user;password=pass

needs format [::]