Yes, Nokia 8.1 does have the HDR10 support and you can watch 4K videos on Nokia 8.1. You can record 4K videos also.
No. Nokia 8.1 is not a water-resistant phone.
Yes, Nokia 8.1 does have the 3.5 mm headphone jack. It also has Bluetooth A2DP support.
Yes, Nokia 8.1 is a dual sim mobile phone.