My understanding is that the four year max is more a consequence of funding sources federal hiring laws than an intentional choice. People get hired into USDS as "excepted service"[0] for a term "not to exceed two years" that can be renewed once. USDS has its own interview process, does its own job listings, has its own recruiting staff, and I think there was no way to do that with permanent, competitive service positions.

[0]: https://www.opm.gov/policy-data-oversight/hiring-information...