Yes, Mi A3 supports dual 4G. You can use both sim cards on the 4G network.
Yes, Mi A3 comes with the latest Corning Gorilla Glass 5 protection.
Yes, Mi A3 does have the HDR support for front & rear cameras.
Yes. You can record 4K videos with Mi A3.
The Xiaomi Mi A3 will only appeal to people who really value the stock Android experience and software updates, and who want good camera performance, without breaking the bank. For that specific target audience, the Mi A3 is currently the best option there is in the market.
The Mi A3 combines a striking gradient design with decent internals and a clean software interface with the promise of quick updates. The 4030mAh battery easily lasts a day, the Snapdragon 665 chipset ensures there's no lag, and the 3.5mm jack is back.