The current build of the BC4 beta does use the current OS user as the SFTP username if a user is undefined, and does use the current .ssh/ private key if no password is defined and Save Password is disabled in the Profile. If you are still having trouble, are you able to use the current Private key file if it is explicitly defined with a saved Profile with a defined username? We've seen some issues with specific public/private key pairs, so this might be an issue with BC4 using the specific key file. We'll need a full copy of the log to help troubleshoot; you can email it to support@scootersoftware.com, along with a link back to this forum thread.

Of note, the username can be case sensitive, which can cause the authentication to fail if the sftp and host OS have differing upper and lowercase characters in the username.

We are a substantially smaller company than Apple, with under a dozen total employees. Our developers are working on both BC3 and the BC4 beta simultaneously, with new builds every couple of months. Not all of the changes are immediately apparent, but many are substantive, such as the complete overhaul of the 'guts' of the Picture Compare in the latest beta build. Plus, fixes for the Samba support that OSX 10.9 broke. Our release schedule trends and development are documented in our changelogs: