Hi.
4bet is fine. Now I'm kind of confused. BU reps a lot of strength with this play. He needs to be worried about you being strong, and SB being strong too.
I also see absolutely no reason for him to slowplay monsters here. Maybe SB might be somewhat aggro? But BB is tight.
I think QQ and AK is probably a call. This could be a call or fold, depending on your image. Let me reiterate my points
1) I see little reason for him to slowplay. SB is a little aggro though
2) Do you fold a lot to 3bets? Maybe that's why he opted to slowplay as well
3) What is your image? Are you known for 4bet bluffing a lot? If yes, this increases the chances of him spazzing here with AQ or TT/99.
4) Is he just spazzy? There are times where I see "standard regs" just randomly shove 88 here. But those times are rare and few. You might want to look out for possible tiltyness - Did he just lost a huge pot?
I'm assuming SB folded? Cos I would snap fold if SB called. I think folding is fine, I wouldn't be too unhappy with a call too.