changed recommended twitch bitrate from 4000 to 3500

According to Cyrus Hall twitch doesn't support 4000 Kbps or up officially.

"We do not officially support streams at 5Mbps (or 4 Mbps for that matter)."

http://blog.twitch.tv/2013/08/new-minimum-broadcasting-requirements-coming-soon/#comment-1001835044