You're correct, it's a new model designation and it appears ASRock does not yet have it listed on its CPU support. However they do have the 5800XT listed (at the bottom) and recommend BIOS 10.10 version.
https://www.asrock.com/mb/AMD/B450M-HDV%20R4.0/index.asp#CPU
Appears newer versions are marked beta so 10.10 is the recommended version. You don't mention what BIOS version you have now, so make sure to read the prerequisites before upgrading. (may need to perform an interim upgrade)
https://www.asrock.com/mb/AMD/B450M-HDV%20R4.0/index.asp#BIOS
Ryzen R7 5700X | B550 Gaming X | 2x16GB G.Skill 3600 | Radeon RX 7900XT
Ryzen R7 5700G | B550 Gaming X | 2x8GB G.Skill 4000 | Radeon Vega 8 IGP
Ryzen R5 5600 | B550 Gaming Edge | 4x8GB G.Skill 3600 | Radeon RX 6800XT