When at layout-tab F4 does browser-preview as is stated on scirra.com/manual/64/keyboard-shortcuts.
To see Expressions panel, you'll need to be at event editor and start adding a new (or editing) an action that needs parameter (for example player sprite: set animation frame number) .. When that dialog box is open F4 open expressions panel.
IMHO it looks kinda unintuitive to use same hotkey to do totally unrelated things depending what part of the program one is. Especially since most of the time it just does nothing at all.