how do i backup my iphone 4 without knowing my passcode ?
Nope, unfortunately for you, one cannot backup his iOS device without knowing the passcode.

The reason likely comes from Apple not wanting backups, albeit passcode-protected ones, made by others who are not authorized by the user to do so. Also, though it would take very long, if the reward was great enough (like cracking a celebrity's or an executive of a rival company's iPhone), one could restore the backup on a same model iPhone running the same version of iOS, guess some passcodes until the iPhone erases itself (as a security measure, iPhones can be set to erase themselves after 10 failed passcode attempts) or the time penalty for incorrectly guessing becomes greater than the time it would to take to restore the backup, the cracker would just restore another backup and guess some more passcodes, eventually guessing the right one.