Average Rating of last 4 inspections/entries.

As long as you don't have too much data, this relativly simple formula which uses a Formula Query will work to have each record flagged as to if it's in the most recent 4.

 So it can be used as a filter in your summary report.

var text QUERY = "{81.EX.'" & [Part Number] & "'}"
& " AND " & "{3.GTE." & [Record ID#] & "}";

Size(
GetFieldValues(
GetRecords($QUERY),3)) <= 4

// replace the 81 with the field ID  of the [Part Number} field