How to use Quarter-Tones
In common western music theory, the minimal distance between two notes is a semi-tone. In other music cultures such as Arabic or Turkish music, quarter-tones can be used. These are the notes whose pitch is in the middle of a semi-tone.
Quarter-tones are a premium feature on Flat. To enable them, you need to go to the instruments settings. Click on "Quarter-Tones", then click on "Enable quarter-tones for this part".
Now that the quarter-tones are available for the instrument, you can find them in the Notes toolbar.
There is a dropdown that allows you to access all four quarter accidentals.