4 drops of water/ antifreeze coming out the resivor hose is not much. But also depends on how full the resivor is to how much is coming out. If the resi is really full thats not much at all. If you can get a hand on a coolant pressure tester you can keep it under pressure via the pressure tester and see if pressure bleeds off. If it does then look for a leak. You can usually drive the truck hard, Let it set over night then pull all glow plugs out. Usually if a cups leaking bad the next day when you crank it over water will shoot out of the hole that a cup is leaking from. I really wouldn't be to worried about the problems that you are describing. The last head gaskets I did on a dmax was puking out coolant about 3 gallons every 10 hours. Head gasket was wasted on both sides! When you start loosing noticable water every day thats when I would be worried. Not just alittle bit of pressure. Hope that was helpful. On edit they also make die for coolant systems. If you feel you have a leak get some at local parts store and find a black light to find external leaks if need be. Most external leaks will be noticable though.
