Can an EA know my MT4 account password by accessing the .DLL file? I mean when I allow to import the .DLL file included with the expert advisor.
What for? Yes it is possible from a dll to get the accounts.ini file and send it to some site or ftp. It is then possible to decrypt the password, but difficult. And for what result? It will only allow to open a few trades with huge volume in order to get a margin call. I don't think it is worth the effort and surely a hacker with decrypting knowledge won't waste his time for such a ridiculous result.
I'm not even sure that other directories than /files/ are accessible, other places are protected, although I'm not very confident with Windows security.
Apart from this, if a malicious person knows your broker account password, not the trading account, it is not possible for him to widthraw money to another bank account with another name. The anti-money laundery rules are very strict and don't allow this.