According to Brother.ch, the machines are identical... according to other review sites, there are differences (see link) e.g. the 40 should have a higher print resolution, slightly faster printing (20 vs. 18 sheets per minute) and has a "multipurpose tray". However, I still have some doubts about these differences as Brother does not list them itself:
https://jonastechknowhow.com/brother-mf...