What stakes are you at mate? I play 2NL and would only defend with any pocket pair, A, K and sometimes any Q, some suited connectors sometimes as well. My 3bet is 11bb and it seems to work more times than it fails. People are generally opening the BTN as a standard steal all the time in the micros but if I get 4bet I would fold unless I had a really good hand to play back with. I don't really think to deeply about the BTN range when they are opening, this might be a leak in my game but it seems to be working for me. This might not be the in depth answer you are looking for but hopefully someone more experienced than me will give a better answer