As from here, I'm using the OpenTopography API to download a geoTiff heightmap, and it uses 4 WGS84 Coordinates, but the map I've supplied to the user gives back longitude/latitude coordinates. I'm very new to GIS, so I was wondering if there was some kind of equation for converting them.
"WGS84" coordinates are (usually) latitude and longitude.
There are lots of different definitions of latitude and longitude, based on different earth shapes or different reference points for 0 longitude. What's called "WGS84" is usually a specific standard for lat-long, more formally called "EPSG Coordinate Reference System Code 4326".
These are the coordinates used by GPS location systems, and should be the default lat-long system for any new data. Sometimes you find older data based on a different earth-shape model or a non-standard 0 longitude meridian. But what I see on that web page API is (99.999%) probably all "WGS 84 lat-long".