1.Have you had any recent repairs done to the cooling system or engine in general?
2.Can you pull over with the engine hot and see any cooling squirting out of the overflow tube?
3. Any smoke at start-up? I mean somebody holding their head right next to the stack and then watching/smelling the exhaust as you fire it up.
4. How long have you owned the truck and how many miles are on the engine?
I've seen engines burn large amounts of coolant and you would never know it by looking at or into the stacks. It usually does not show up. I've also seen engines burn large amounts of coolant without the slightest indication of any sodium/potassium in the engine oil also. I have a few theories on this but they don't really matter. 4 gallons in 300 miles is a lot.
1. Do the above test with a bottle to see if it is in fact going out the overflow hose. Usually the bottom of the truck will have coolant all over it too.
2. The next thing I like to do is disconnect/block off every external source of coolant flow to/from the engine. Shut off heater lines. I have taken the air compressor completely off and installed a block off plate before. You just want to isolate the problem to either the engine or the truck so you're not wasting time, chasing ghosts.
Once you completely isolate the engine from everything but the radiator, I verify if it actually is combustion gas by making a home made radiator cap (I used an adapter from an old cooling system pressure tester) with a manual pressure release valve on it and a 15-20 lb pressure gauge. Run the engine to operating temp-bleeding off the air pressure about 4 times to remove the natural pressure formed from coolant expansion- then see how high the gauge will go (Don't go much above 15 psi or hoses may start blowing). If the gauge keeps rising after bleeding it off after about 4 times, you will have 100% confidence that combustion is leaking into the cooling system and coolant is probably being sucked back into the cylinder on the intake stroke.
This leads to tearing into the engine.
These problems, I've found, are usually never easy to find.
What I usually do next is pressurize the radiator (with the engine cool) to about 10 psi, then remove the oil pan, all 6 injectors, and any cover on the front of the engine and search for the coolant leak with a high quality borescope. Even if it takes all day, a good, detailed search will usually locate the coolant and then maybe lead to your problem.
Almost every time I've seen this happen on a Caterpillar, and everything else externally has been checked/changed, I remove the head and carefully inspect all 6 fire-rings on the old head gasket. The fire ring is the solid metal ring on the inside of each of the 6 circles. (A lot of shops pitch the old gasket without inspecting it). There will be a small black "soot" mark usually about 1/16" inch in one or two corners of one, two or three fire rings. That soot is where the combustion is leaking into the cooling system. (If that's the problem). It will at least lead you to inspect that particular spot on the deck or head for abnormalities or warping.
Head gaskets usually only blow when the engine gets very hot (the metals expand), or the liners slowly fall down into the cylinder block (usually higher mileage). If the shop says you have a blown head gasket, yet you never overheated the engine, MAKE SURE they check the "liner-protrusion" when the cylinder head is removed.
Give us an update and I hope this helps a little.