I don't know what a Breville TT4 toaster is, but when my toaster wouldn't engage in the "down" position, I took it apart and discovered the engaging mechanism was actually what appeared to be an electro-magnet. This explained why it wouldn't engage when the toaster was unplugged. I simply buffed up the magnet and the part that makes contact with it with some fine sandpaper and it worked fine for about 3 months. Now it won't stay down again and my magic fix didn't seem to work this time. I'm in Africa and toasters here are in the $40 - $60 range so I'd like to repair instead of replace, but I don't know what else to do. I hope this works for you.
Special K, February 2010