The Marker Guage
visualization expects to be passed a single value and it does not care what the field name is. In order to do this for your 4 hosts, you need to make 4 separate panels which should contain your first search as-is but end like this:
... | search Appname=A1 | fields Until%
... | search Appname=A2 | fields Until%
... | search Appname=A3 | fields Until%
... | search Appname=A4 | fields Until%
Put the rest of the context to explain the Hostname, etc. in the "Title" of each Panel.