Take a look at Find/Replace in the edit menu. You can read more about this tool in the help system.
If it's only 4 fields, create a 'Find' script, and offer a global field for the user to enter the criterion. Then when they hit your 'Find' script button, create a Find Request and set the global value into Field1, create a second request and set the global value into Field2 etc, then Perform Find.
Alternatively, create a 5th Search Field which is a concatenation of the other 4 fields, with spaces between the values: Field1 & " " & Field 2 & " " & Field 3 etc. Filemaker will match to any of the values in that space-separated list.
Thanks for this. So simple, yet so impossible to find when looking at FM help.