How large is your Photos Library? The updating can take a very long time for a large library, without the status bar showing any progress. For example, for my main library with 200GB it took three hours.
If your library is similar large, let the update run for a few more hours.
In the meantime you may want to check, if there is a problem with your library location. Is your Photos Library in your home folder on your system drive or on an external drive? Photos 4.0 may not be able to work with a library on an external drive, if the drive is also used by Time Machine, or incorrectly formatted. An external drive should be prepared like described here: Move your Photos library to save space on your Mac - Apple Support