After some unspecified sort of cosmic phenomenon occurs, all the ants in a particular region of Arizona start getting smart. Really smart. Two scientists are sent to investigate, setting up a geodesic dome stuffed with what counted for high tech in the 70's. And the fun begins.
This movie was directed by Saul Bass, who is perhaps most famous for designing the title sequences to some of Hitchcock's biggest films, including Psycho. Phase IV feels almost exactly like a moderately clever sci-fi short story put to film. The best thing about it is that the ants are suddenly building structures with right angles and geometric designs, digging perfectly straight rows of perfectly round holes. That in particular was effectively weird. There were certain exterior shots of such structures that had the same alien feel (and indeed, some of the same ambient sound effects) as the animated movie Fanstastic Planet. There are notably few special effects at work; all the ant action is handled with footage of actual ants, several hundred of which were probably harmed in the production. I think they even injected some live ants with dye.
All in all the story holds itself together nicely, delivering the kind of mild, slightly stylized stinger ending you'd expect of a decent short story. And, because it's Saul Bass, there's a clever thing going on with the credits. A decent entry.