I just changed mine - twice - without any problems.

Are you deleting the old password before trying to enter the new one? You can't just add letters on to the existing password.

I just tried this - if you don't delete the old password, and just type a new password in front of it, then you get the error message you describe, even if the combined length of the two passwords is between 4 and 20 characters. For example, if your old password is "drunk" and your new password is "sober", and you don't delete the old password, you would think your new password would be "soberdrunk" which is between 4 and 20 characters, but it gives you the error message anyway.)