This Yuil Ban thread - Foundations of the Fourth Industrial Revolution - explains it best. While I recommend reading the entire thread, if you don't want to, here are some quotes:

"The Fourth Industrial Revolution is the upcoming/current one. And this goes into my second point: we won't know when the Fourth Industrial Revolution started until WELL after it's underway.

Next, "inter-revolutionary period" refers to the fact that technology generally progresses in inter-twining S-curves and right as one paradigm peaks, another troughs before rising. This is why people between 1920-1940 and between 2000 and 2020 felt like all the great technologies of their preceding industrial revolutions had given way to incremental iterative improvements and great laboratory advancements that never seemed capable of actually leaving the laboratory. If you ever wondered why the 2000s and 2010s felt indistinguishable and slow, as if nothing changed from 1999 to the present, it was because you were living in that intermediate period between technological revolutions. During that time, all the necessary components for the Fourth Industrial Revolution were being set up as the foundations for what we're seeing now while simultaneously all the fruits of the Third Industrial Revolution were fully maturing and perhaps even starting to spoil, with nothing particularly overwhelming pushing things forward. You might remember this as "foundational futurism."

As it stands, a lot of foundational stuff tends to be pretty boring on its own. Science fiction talks of the future being things like flying cars, autonomous cars, humanoid servant robots, synthetic media, space colonies, neurotechnology, and so on. Sci-fi media sometimes set years for these things to happen, like the 1990s or 2000s. Past futurists often set similar dates. Dates like, say, 2020 AD. According to Blade Runner, we're supposed to have off-world colonies and 100% realistic humanoid robots (e.g. with human-level artificial general intelligence) by now. According to Ray Kurzweil, we were supposed to have widespread human-AI relationships (ala Her) and PCs with the same power as the human brain by 2019. When these dates passed and the most we had was, say, the Web 2.0 and smartphones, we felt depressed about the future.

But here's the thing: we're basically asking why we don't have a completed 2-story house when we're still setting down the foundation, a foundation using tools that were created in the preceding years.

We couldn't get to the modern internet without P2P, VoIP, enterprise instant messaging, e-payments, business rules management, wireless LANs, enterprise portals, chatbots, and so on. Things that are so fundamental to how the internet circa 2020 works that we can scarcely even consider them individually. No increased bandwidth for computer connections? No audio or video streaming. No automated trading or increased use of chatbots? No fully automated businesses. No P2P? No blockchain. No smartphones or data sharing? No large data sets that can be used to power machine learning, and thus no advanced AI.

Finally and a bit more lightheartedly, I'd strongly recommend against using this to predict future industrial revolutions unless you're writing a pulp sci-fi story and need to figure out roughly when the 37th industrial revolution will be underway. If the Fourth Industrial Revolution pans out the way I feel it will, there won't be a Fifth. Or perhaps more accurately, we won't be able to predict the Fifth, specifically when it'll take place and what it will involve."