Bootstrap v4 uses Flexbox, which is supported by almost all modern browsers, so you are unlikely to encounter major compatibility issues with Bootstrap v4. However, if you want to support older browsers, you will need to use polyfill or fallbacks for some new CSS properties implemented by Bootstrap v4.
Here's the list of supported browsers for v4:Chrome 45 Firefox 38 Opera 30 Internet Explorer 10 Edge 12 iOS 9 Safari 9 Android 4.4
Bootstrap 4 supports mobile version browsers:Android• Supported in chrome • Supported in Firefox • Supported in Microsoft Edge • Not Supported in SafariiOS• Supported in chrome • Supported in Firefox • Supported in Microsoft Edge • Supported in SafariWindows 10 Mobile• Not Supported in chrome • Not Supported in Firefox • Supported in Microsoft Edge • Not Supported in SafariBootstrap 4 supports desktop version browsers:Windows• Supported in chrome • Supported in Firefox • Supprted In IE10 and above • Supprted In Microsoft Edge • Supprted In Opera • Not supported in SafariMac• Supported in chrome • Supported in Firefox • Supprted In Opera • Not supported in Safari