I still can't help looking at the situation and thinking that actually I'm looking at this particular bet resulting in a total investment of 6 bb (my 4 bb from pre-flop and the 2 bb needed to call on the turn)
Your pre-flop bet is already in the pot, and is no longer part of your investment (ie: it's no longer yours) since you don't have the option to remove it from the pot when you fold. Hope that helps.