cmake 4 (currently in testing) removes support for cmake_minimum_required values below 3.5. This may cause build failures especially for unmaintained or poorly maintained upstream projects. Those can be fixed by increasing the cmake_minimum_required value in CMakeLists.txt. Note that this will change the default value of some policies so may cause some behavior change. Also, OLD vaue for some deprecated policies is no longer supported. If whatever the policy's OLD behavior does is still needed, it will need to be set in some other way. Check the specific policy's documentation for more info.