Bootstrap 4 no longer bundles (a subset of) glyphicons. Instead, Bootstrap recommends to buy the full set or use a different set of icons.
However, if you want to quickly copy-and-paste some stuff over from a legacy Bootstrap 3 project, the easiest of getting the old icons is through loading bootstrap4-glyphicons. It’s a project that only contains the glyhpicons from Bootstrap 3.
Once installed link to its CSS file and you’re good to go.
<link href="/bootstrap4-glyphicons/css/bootstrap-glyphicons.min.css" rel="stylesheet" type="text/css" />