I assume the FTP server password changes if you change the box password (although I don't know). But I don't think many people run an intrusion detection system on their home network to detect someone trying all 10,000 four digit passwords, which would probably take less than a couple of hours.
Entirely up to you, of course. Someone would have to portscan your address, recognize it as a Humax (I don't know if FTP servers announce their software), and run through all the passwords. All while the server is on. How big the risk is depends on how long you leave the access configured for, how much time you spend with the Humax box on, etc. Personally I wouldn't take the risk but some might.