that does call the validaty of it being a serious military-political alliance in question
V4 is NOT a military-political (that's NATO) alliance but an economic-political alliance. It was created in 1991, way before any of our countries applied for EU membership and nobody wants to leave it to this day. Not bad for an irrelevant
organization, right? And it got stronger after 2015 "wilkommen politics (btw. it amazes me is how relatively few people fully realize the importance and political implications of 2015 Merkel's decision. I've got no doubt this date will mark an important in future history books).
Another strong point of V4 is that it doesn't change the rules in the middle of the game and it doesn't try to rob any of its members of political sovereignty.
Remember that the EU had a humble beginning as a "European Coal and Steel Community" so don't underestimate the cooperation between CEE countries with a similar culture, history, and economic development.
3 Seas looks similar but worse
And that's where the US steps in to act as an equalizer. So the whole initiative would not be sunk by the EU core (or without sugar-coating: by old German plans for mitteleuropa btw. I have to admit that I admire such a consequence) before it even takes off. And that's why I think that an initiative that could not work in the past (like pre-war Intermarium) can work now with an active US involvement in Europe.
3SI is correctly seen as a threat, or at least a major disruption, to a Franco-German political dominance Europe. Polish politicians diplomatically deny it, but let's be honest: it is not a brainchild of the EU, and it takes away a chunk of control from the Berlin-Paris-Brussels axis. But that's good, Europe needs real diversity and some competition between different politico-economic centers.
It was created just a few years ago and there are some large economic projects, mainly initiated by Poland, but serving the whole group: gas pipelines, Via Carpatia expressway connecting almost all the countries from Baltic to Mediterranean sea, 3SI investment bank, and so on.