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If we speak about installing Firebird client only, you need to have fbclient.dll file. Firebird 4.0 client requires Microsoft Runtime C++ 2017 with the same bitness as fbclient.dll. If Microsoft Runtime is not installed, you may just copy it’s two files, msvcp140.dll and vcruntime140.dll that are included in ZIP for Windows.
Also, it is welcome to have firebird.msg at the same place where fbclient.dll is installed or copied. There are lot of error messages included in fbclient.dll, but at least for Firebird console tools it is good to have`firebird.msg` nearby.
Unlike Firebird 2.5 and 3.0 Firebird 4.0 client library may need ICU files (icudt63.dll, icuin63.dll, icuuc63.dll и icudt63l.dat). Previous versions required these files only for the server, not client. Now these files may be needed by client library, especially if you want to work with datatypes like TIMESTAMP WITH TIME ZONE and TIME WITH TIME ZONE. ICU is also used by Firebird client for UtilInterface::decodeTimeTz() and UtilInterface::decodeTimestampTz() functions.
Note: Windows 10 already have appropriate ICU library.