Changed keyboard shortcuts in Photos 4.0 on Mojave

Some keyboard shortcuts have changed in Photos 4.0. The spacebar shortcut is now used to start and stop a video or a slideshow.

See the updated user guide: Photos keyboard shortcuts on Mac - Apple Support

Open or close a photo: Double-click a photo thumbnail or select a thumbnail and press Return. Double-click or press Return again to close.

The shortcut "R" for the Retouch Tool is also gone, but without a replacement.

To be able to use the spacebar again to open a photo, see Eric in Boca's post this discussion: Re: Space bar won't open photos in Mojave (read the complete discussion, if the fix does not work at once):

Go to preferences > keyboard > shortcuts > app shortcuts > Photos > and add a shortcut. For Menu Title enter "Open Viewer" and for the keyboard shortcut press the spacebar whilst holding the Fn key. Add another one for "Close Viewer". Problem solved