Originally Posted by Diesel_Bomber
Check your battery cables and connections. You should have bright shiney metal everywhere.
I've seen battery cables that look pristine on the outside be full of nothing but green goo. Slice open the insulation on your cables near the end and inspect the conductor itself - again it should be bright shiney copper. Obviously you'll want to seal the cut well after this, or you might cause exactly that problem in the future. You can also try and start the truck and then feel the cables for warm spots.
You can also try swapping in a known-good battery to see if your starting problem magically goes away.
I've had starters, alternators, and batteries all test good, yet fail in service. A replacement cured their ills.
Edit: At 4 years, your battery is probably toast. Seems like there's not much warning anymore - a vehicle starts fine one day, next day the battery is a doornail. I've taken to replacing them at three years, regardless of how they behave.
All good advise --
I would have done more testing if the battery was not 4 years old.
I was surprised to discover that same battery that goes in a 2.5L 5 cylinder VW Jetta (gas) is also the spec for my V8 Chevy Tahoe.
I considered doing a swap - but the dam Tahoe battery is a PITA X4 to remove.