Using 9mm diameter semi static climbing rope as an example. For volume: Volume of 4km of rope is 0.009^23.144000 = 1.02m^2 which seems like an amount that woul

Using 9mm diameter semi static climbing rope as an example. For volume: Volume of 4km of rope is 0.009^23.144000 = 1.02m^2 which seems like an amount that would fit on a large spool on a ship. For weight: rope itself weighs about 60g per meter so 0.06*4000=230kg of rope. It has a breaking strength of 22kN, which is roughly 2000kg static load. Given everything is probably roughly neutrally buoyant seems like enough. Not saying its practical but it seems like the actual volume/weight of rope would not be a problem.