C terminal is best served by central parking. It is $24 a day. It is the most convenient for late arrivals, but you pay for the privilege.
You can walk to C from the B garage, but right now the B garage and arrivals/departures area is under construction and about 30% of the spaces are blocked off due to the construction. The C garage is a massive, 7 story structure with two halfs. If you go straight after entering the garage, that takes you to the half that connects to C via moving walkway on the 4th level.
For a 4 day trip, the price penalty may be worth it for convenience unless you live near a Logan Express location and that location has buses up to midnight in case your arrival is delayed a bit.