I am still ironing out a few stubborn bugs in FotoSketcher 3.80 (a new beta should hopefully be available soon), but in the meantime here is a small tutorial on how to use Photoshop® .abr files in FotoSketcher.
1/ First, you need to find .abr brush files, which are avalaible for free from many websites. You can for instance head over to https://creativenerds.co.uk/category/photoshop-brushes/
2/ Then unzip the file you just downloaded and locate the .abr file on your drive
3/ If the .abr file contains only one brush, you can simply convert it to a transparent png image using an online service (no need to install anything), like https://www.coolutils.com/online/ABR-to-PNG
4/ Alternatively, if the .abr file contains a set of different brushes, the easiest way to convert them to png images is to use some free software, like ABR Viewer or ABRMate (download for free from https://abrmate.software.informer.com/ and install on your PC)
5/ Finally, in FotoSketcher, choose Painting 4 and click on "load custom brush" and choose one or multiple png brush files that you just converted
Have fun :)