Make sure the engine is tilted back so the manifolds drain (raise front of trailer if you have one, otherwise pour antifreeze down the hoses that go into the manifold. Pull off the hose to the engine water pump and drain that. Pour some pink antifreeze into the hose going to the engine water pump until it comes out the thermostat housing (where the hose was connected). Pour some antifreeze into the hose coming from the stern drive until it runs pink out the stern drive (this gets the power steering cooler too). and you should be good to go. My 1989 4.3 only has four drain plugs.
Some power steering coolers have a drain plug, perhaps that is your fifth. The PS cooler is by the engine mount on the port side of the engine. It is just a thick part of the hose system that comes up from the stern drive.