With the release of Bootstrap 4, a few components have been dropped and a couple of new ones have been added. The most significant change is the new Cards component. Let's start by breaking down this new option.
With the release of the Cards component, the Panels, Thumbnails, and Wells components have been removed from Bootstrap 4. Cards combines the best of these elements into one and even adds some new functionality that is really useful. If you are migrating from a Bootstrap 3 project, you'll need to update any Panels, Thumbnails, or Wells to use the Cards component instead. Since the markup is a bit different, I would recommend just removing the old components altogether, and then recoding them using the same content as Cards.
The GLYPHICONS icon font has been removed from Bootstrap 4. I'm guessing this is due to licensing reasons as the library was not fully open source. If you don't want to update your icon code, simply download the library from the GLYPHICONS website at:
The other option would be to change the icon library to a different one such as Font Awesome. If you go down this route, you'll need to update all of your <i> tags to use the proper CSS class to render the icons. There is a quick reference tool that will allow you to do this called GlyphSearch. This tool supports a number of icon libraries and I use it all the time. Check it out at:
http://glyphsearch.com/
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Those are the key components you need to be aware of. Next let's go over what's different in JavaScript.