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...

At the time, I only found a reference at Druckerchannel at the bottom of the page that they are identical in construction (https://www.druckerchannel.de/artikel.ph...).

Event. Memory cards and connections. Otherwise both are identical