Bootstrap 4 is 30% smaller than the latest Bootstrap 3 build! It previously was around ~123kb and now is a mere ~88kb. This is a huge reduction for not losing any features.

• Moved from Less to Sass
  
• Improved grid system
  
• Opt-in flexbox support is here
  
• Dropped wells, thumbnails, and panels for cards
  
• Consolidated all our HTML resets into a new module, Reboot
  
• Brand new customization options
  
• Dropped IE8 support and moved to rem and em units
  
• Rewrote all our JavaScript plugins
  
• Improved auto-placement of tooltips and popovers
  
• And tons more!