If the 4x4 is equipped with an automatic transmission and the coolant has turned a milky pink, the transmission cooler is probably leaking. Engine oil is a clear amber when new, though sometimes it appears a smoky olive. Under normal conditions, the color will darken to a golden brown after a few hundred miles of use. Brown or black stains underneath the front half of the truck are oil. If your 4x4 has a manual transmission, you may see a leak under it as well.
Oil leaks are most noticeable on older trucks, and are mainly due to component wear.
Other common causes are worn gaskets, faulty seals, or even a loose oil filter. Since the engine holds several quarts of oil, it won't miss small drops. But a sizable stain requires immediate attention. If the oil appears very dark even though you change it frequently, combustion products may be working into the crankcase through worn piston rings or a damaged cylinder. If the oil has a frothy or milky appearance, it may be contaminated with coolant, probably via a blown head gasket or a cracked engine block.