Exact concurrent player counts for 2019 are not publicly archived in a single definitive source, but we can reconstruct a reliable picture using platform data.
Steam provides the most transparent historical data. In 2019, Fallout 4 consistently ranked in the top 50-100 games by concurrent players.
Source: SteamCharts historical aggregates for AppID 377160.
Sony and Microsoft do not release public historical concurrent user stats per title. However, Fallout 4 was a perennial best-seller:
Console active user bases in 2019 were likely comparable to or larger than PC, but split across two platforms.
The Bethesda launcher (discontinued 2022) hosted a significant PC population for the Creation Club and mods, uncounted by Steam.
Summary: In 2019, Fallout 4 likely had 50,000–100,000+ concurrent players across all platforms during peak hours, with a monthly active user base in the millions.