I'm trying to export an Esri GRID file From Reefmaster into ArcMAP 10.4, but when I try loading the .asc file, I get an error saying the spatial reference could not be found. any idea what I am missing or need to do to get this to work?

As far as I know, ESRI ASCII grids do not contain any information on the datum or projection, so this is something you probably need to specify during import. Unfortunately we don't have ArcGIS here so I can't test this for you. Perhaps someone else here can help?

By reading the GDAL manual page www.gdal.org/frmt_various.html it seems that projection could be captured from .prj file. I believe that for EPSG:4326 the contents of the .prj file should be these epsg.io/4326.esriwkt.