The GMC TopKick had a really good run with the Transformers franchise, surviving through the end of the third movie on the strength of Ironhide's close friendship with our hero.
But General Motors quietly exited the C4500 - C8500 truck classes altogether in 2009. Most of the larger trucks were absolutely ancient. Imagine a Frito-Lay extra-long delivery truck, and you've got it.
The TopKick was a strong option for the films: huge, versatile with a dump-truck rear end, and looked great.
Ironhide outlived the Hummer H2 BayWatch car and Jazz, the oft-mocked Pontiac Solstice character with a somewhat racist accent.
Ironhide died in battle during Transformers 3, and will be replaced by a new medium-duty truck named Hound.
Hound wants to be human, which is an endearing trait for any machine, I guess. The legendary Oshkosh Defense FMTV gets a bad-ass jungle paint job, heavy duty roll bars, and hundreds of other special effects tricks.
From my research, it looks like the 6x6 Cargo variant of the FMTV family. More likely, however, is the 10-Ton Dump Truck version. The model numbers are below, along with a variety of other heavy tactical trucks from Oshkosh -- which look mean enough to be Decepticons...
"Hound" - Transformers 4 previews from MichaelBay.com
Oshkosh FMTV 6x6 10-Ton Dump (M1157 A1P2)
Light Trucks from Oshkosh
Heavy Trucks from Oshkosh