The storyline from Part 3 continued on into the TV series, so Part 4 instead tells about how the Graboids first showed up in Perfection Valley way back in 1889. It seems that hot water that was runoff from the new silver mine uncovered and incubated some long lost Graboid eggs and they hatched. The little baby Graboids killed a bunch of miners and it shutdown due to workers refusing to reenter the mine.
A few weeks after the closure the owner, Hiram Gummer, travels from Philadelphia to personally see what's going on and what he finds is a dying town. Everybody's split and the only ones left are the Chang family, a feisty red-haired owner of the local hotel and a few odds and ends people. City-slicker Gummer isn't accustomed to the rough country life, but he soon adjusts and investigates the silver mine and finds it infested with baby Graboids. Barely making it back to town alive he hires the help of a bounty hunter and even buys a bunch of guns and quickly discovers that he really, really likes guns.
I like this movie. The budget is respectable, there's less CGI than in Part 3 and the awesome full-grown adult animatronic Graboids are back. I also liked the story and enjoyed seeing the evolution of the town and the transformation of Gummer from city boy to rough gun-toting outdoorsman. My only complaint is I would have liked to have seen some more returning cast members, but I guess that was just impossible. If you like the TREMORS series then this film is a must watch.
Part 1 - Tremors
Part 2 - Tremors II: Aftershocks
Part 3 - Tremors 3: Back to Perfection