Average Rating of last 4 inspections/entries.

Here is a brute force method.     Build this

The field Pipeline count you should set to a Default Value of 1.   Everytime an Part Inspection record is created it will be set to one.

Make a Pipeline that fires when Part Inspections are Created

Step B is a Search of the Part Inspections Table

Your advanced query should be that Related Part = a.related_part and

Record ID not equal to a.id

Then Step C is an Update Record

Pipeline Count +1

In your Summary Report filter your inspections where Pipeline Count <=4

You will need to clean up your old data but going forward only the four most recent Part Inspections will be used in the Summary Report.