Nowadays major games have multiple leads in various categories because game development has become so bloated. Just like the addition of a lead artist in 2019, looking for a lead producer doesn't mean that there is a bottleneck in development where nobody can do much work on the game until the position is filled.
Retro Studios got assigned with the development of Metroid Prime 4 only ~20 months ago, so the game isn't close to release regardless of who they hire or not. I say this because I commonly see people talking about development hell or vaporware in regards to MP4 when Nintendo was actually upfront about the game getting started from scratch again after the initially assigned developers couldn't produce satisfactory results. Retro didn't pick up a halfdone game, they started over instead.
The first Metroid Prime took ~2.5 years to develop, the second one took 2 years (they had the engine and made a sequel on the same console), the third one took ~2.5 years again (engine could be reused for the Wii, but controls had to be reworked, so extensive additional testing was necessary). Given that game development has only got more time-consuming, 3 years is the absolute minimum that must be expected, so a late 2021 release of Metroid Prime 4 is the most optimistic expectation one can have while still maintaining a tiny sense of realism. More reasonable is a release in 2022, though.
Given what MP4's history has been so far with its early announcement at E3 2017 due to Samus Returns on the 3DS (in 2017 Nintendo still had to assure fans that Switch is their clear priority) and its restart of development that was announced in January 2019, Nintendo is unlikely to show the game until its close to completion. So if anyone expects to see something of it within the next 12 months, they are setting themself up for disappointment.