Sorry, the data is security data, so can't really provide samples. This didn't work exactly either, but I think i've got it now, based off this answer. I really don't like it, and i'm sure that there is a much better/cleaner way to do this.
I've changed around my format of the CSV a little, and have pulled the field5 out all together, and I also pulled the second "matching" line out. so my sample CSV looks like this now:
Summary Field1 Field2 Field3 Field4
Event1 foo1 foo2 foo3 foo4
Event2 bar1 bar2 bar3 bar4
etc
my search is:
sourcetype=mysource
[ inputlookup mycsv.csv
| fields field1,field2,field3,field4 ]
| eval field5Exists=if(isnotnull(field5),"TRUE","FALSE")
| lookup mycsv.csv field1
| eval Summary=if(like(field5Exists,"TRUE"),"Event That is no longer in CSV","Event1")
| table _time,Summary
... View more