The glyphicons.less file from Bootstrap 3 is available on GitHub. https://github.com/twbs/bootstrap/blob/master/less/glyphicons.less
It needs these variables:
@icon-font-path: "../fonts/";
@icon-font-name: "glyphicons-halflings-regular";
@icon-font-svg-id: "glyphicons_halflingsregular";
Then you can convert the .less file to a .css file which you can use directly. You can do this online on lesscss.org/less-preview/. Here I've done it for you, save it as glyphicons.css and include it in your HTML files.
<link href="/Content/glyphicons.css" rel="stylesheet" />
You also need the Glyphicon fonts which is in the Bootstrap 3 package, place them in a /fonts/ directory.
Now you can just keep on using Glyphicons with Bootstrap 4 as usual.