I think most of the lines available would be correct for different table reads but given that you have no read and no image it's a really tough spot imo. For example, it's hard to know if people are going to respect a 4bet here, or if they will flat with worse or even 5bet weaker value or with air. But without reads I guess it's best to assume that most people are playing reasonably straight forward, obv more agg than normal though given that it is 6max. So I would assume that if you cold 4bet to eg 350 here then you are probably folding out most worse hands, maybe getting flats from some mid-pairs, big aces etc, though they will be suspicious and probably not paying off more than 1 street without hitting. If you were to 4bet and they 5bet you then their range is likely crushing you and I think without reads calling off would be bad there, though AK will be in some people's ranges and bluffs for some too. So I don't really like 4bet/calling or 4bet/folding and think that the times you do get flatted you are probably not going to get value. Which makes me think that flatting is the best line, disguising your hand and letting you pot control a bit too.