I can remember from previous discussion on the old TD forum that it was agreed collusion or soft play is not something that can be defined by a rule book, instead must be ascertained by the ruling TD at that particular time. In this case, yes IMO it is soft play and I would give a penalty had I not been at the table, When down to 4 players in a 60 strong tourney I would ask though where else would I be!? Or if I was not there one of my Floors would of been at the table, and would of ordered the BB to call regardless and then give a penalty.
Regards
Stuart