You can use the forgot password link to make a new password.
Or alternatively if you are logged in you can do it through your profile page Settings > Security > Change Password.
What about signing in in MT4 from google? Is it possible?
(i'm talking about the desktop version)
No, you need a valid demo or real account login details for that.
Don't confuse MT4/5 that are trading terminals with MQL5.com which is not a broker and does not offer real trading accounts.