Historically you are correct that only around 4,000 employees of the US federal government are 'political hires' that the President can change/hire/fire. However in October 2020, Trump ordered Schedule F [0] that effectively increases the number of career bureaucrats that could be made by political appointment rather than by a competitive process. The Order was promptly rescinded by the Biden administration in Jan 2021. [1]

The Trump administration said that they believed Schedule F would increase the number of career positions to roughly 50,000 existing jobs. However many think tanks and union members believe that Schedule F could be interpreted much more broadly and could include well over 100,000 positions.

If the Trump administration revives the Schedule F order, it could mean very significant changes for many career bureaucrats.

[0] - https://www.govinfo.gov/content/pkg/FR-2020-10-26/pdf/2020-2... [1] - https://www.whitehouse.gov/briefing-room/presidential-action...