FlameRobin search for fbclient.dll in the wrong place for Firebird 4.0 - there is no BIN directory in Firebird 4.0 install folder

In Firebird 4.0, all executables are located in the root directory of its installation, including fbclient.dll.

As a temporary fix, you can create the symlink to the BIN folder:
  mklink /D "C:\Program Files\Firebird\Firebird_4_0\bin" "C:\Program Files\Firebird\Firebird_4_0"