4MOST is designed to address many science cases, and millions of spectra will be obtained for most of the surveys. This will have a huge legacy value for the community, in part due to a large overlap with existing and future observational facilities. In particular, 4MOST will provide the spectroscopic complements to the large-area surveys coming from key European space missions like Gaia, eROSITA, Euclid, PLATO, and from ground-based facilities like VISTA, VST, DES, LSST, and SKA.
The most straightforward source of overlap with other surveys comes from the target catalogues of Consortium Surveys, which are constructed using different existing and planned observations, especially in combination with future data releases by the Gaia mission. For more details on how they produce their catalogues and what is their expected footprint can be found in the respective White Papers.