Originally written in 1983, thus prior to the Internet, cellular phones, and plenty of other luxuries we take for granted in the 21st century, I'll say this: Nourse was nothing if not prescient when it comes to how the United States (as a whole) would behave if a world-wide pandemic suddenly crapped itself into our laps. In The Fourth Horseman, the terror is a resurgent strain of Yersinia pestis, the bacterium responsible for both the first recorded pandemic in history (starting in 541CE and lasting for over two centuries) as well as the Black Plague, the well-known 14th century bubonic variant which ran rampant for a mere seven years, yet wound up the single most catastrophic pandemic known to humanity.
Guess we're lucky in that we're "only" dealing with a coronavirus this time around. Maybe it's weird reading about a pandemic during a pandemic, but hey, sometimes you gotta do what you gotta do.
Nourse's prose reads very quickly, and this is a good thing. I could see a doctor (the author was also a physician) getting bogged down in the details, and Nourse does occasionally have a character do an info dump for the reader's benefit, but the story keeps moving as Nourse follows small groups of protagonists in different spots around the country, all of whom are trying to get a handle on things locally before things get out of hand. Naturally this is all but impossible, but they have to try anyway. Not all of them make it in the end either, and by the end of the book, the fates of a number of them are up in the air. Despite this, Nourse takes pains to paint the devastation as realistically and accurately as possible, to explain what an outbreak of plague would look like in the late 20th century, and to show that with the right planning, precautions, and preparation, whether you're a pharmacologist or a down-home country doctor, it is possible (though not guaranteed) to come out of the other side.
The scenario is compelling, the writing is competent, and Nourse's prognostications about how society at large would respond to such a pandemic hit with painful accuracy, with some people pulling together and doing their best to stop it, while others shrug their shoulders on the assumption it's not that big a deal and wind up making things worse. Ideas like contact tracing, masking, and isolation are all used to try and counter the beast. But, of course, people being people manage to ruin everything, whether it's rioting and looting in the face of lockdowns, or ignoring their symptoms and exposing more people, allowing the disease to spread further and faster. Some things never change.
Anyway, all things considered, this was an enjoyable read. The ending comes on a bit too abruptly for my liking, and as mentioned, there are a number of POV characters whose plotlines are left unresolved after the last page, but I quite liked this and would happily recommend it to anyone who likes a good medical horror thriller in the same vein as films like Outbreak and Contagion.