Which seems kind of odd. After all, while the term got started in video games, it's been around in practice in board games since at least the 70s.
The term refers to Explore, Expand, Exploit and Exterminate. Exploring means finding out what the map is. Expand means creating some kind of infrastructure, usually settlements and the like. Exploit means using the resources you find, almost always involving a tech tree. Technically, Exterminate means fighting and eliminating other players but it seems to just mean fighting these days.
As a genre, 4X really seems to have taken hold in the video game arena, which is where the term was coined in the first place. Basically, we're talking about in depth empire building. The term usually implies a big scope and a high complexity.
Over the last few years, what seems to have become a goal is create shorter and more accessible 4X board games. The first time I really heard the idea bandied about was with Eclipse, which described to me as Twilight Imperium for people who only want to play Euro games.
When it really struck me was when Tiny Epic Kingdoms, a 4X game that can fit into a really big pocket and take about a half hour to play. And now there's even a travel version that will fit into just about any pocket.
And when I got a chance to play Scythe, it was described to me as a 4X game that combined the sensibilities of Ameritrash and Euro. It sure seemed to live up to that description.
It seems to me that the term 4X might be changing or shifting, making it harder and harder to definitively pin down what is and what is not a 4X game. In particular, the Explore and the Exterminate aspects seem more nebulous to me.
With computers, the ability to create new maps and ones that are filled the fog and the unknown is a lot bigger than what you can do with a board game. Heck, in Scythe, a major component to the exploration experience are encounter tokens, which are one-shot multiple choice benefits.
And, as I already commented, Exterminate originally meant just that. Wiping someone off the map. Now, it seems to just mean fighting. With a loose enough definition of fighting to include siccing the robber on someone, someone could argue that Catan is a 4X. Not that I think anyone could actually get away with that argument.
I got curious and looked up if Nexus Ops was considered a 4X game. And I found that it was not. While it does have exploration and exploitation and even extermination, it does not have expansion. Which makes sense. Your forces really just squat on mines. You don't build any infrastructure.
Ultimately, what I'm getting out of 4X becoming a more common term is that the term has gotten more vague and not everyone is going to agree on what qualifies.