I’m honestly at a bit of a loss on the issue with your post grid slider. What I can tell you is that you’re looking in the wrong place. The builder is saving the 4 columns just fine, and when you look at the frontend of your site, you can see that the 4 columns are indeed getting inserted, but the problem is something to do with how the width is getting calculated on the slider plugin. For whatever reason, your post grid is being stretched really wide, and so you can’t see the other 2 columns.
You have so many plugins and things stuffed into head of your site. You need to see if there is some sort of plugin conflict or possibly a conflict from some sort of custom CSS you’ve put into the theme. But something is messing with that slider being rendered that isn’t part of the theme (unless you’ve modified something that I’m not aware of, which doesn’t sound like you did). This conflict is not allowing the FlexSlider to calculate the widths correctly, and you’ll notice it can’t even rotate slides because of this.