user: Question: What is the average episode number where Jimmy Mulville was the 4th performer?
Columns: ['Date', 'Episode', 'Performer 1', 'Performer 2', 'Performer 3', 'Performer 4']
Types: ['text', 'real', 'text', 'text', 'text', 'text']
Sample row: ['2 January 1988', 1.0, 'John Sessions', 'Stephen Fry', 'Dawn French', 'Lenny Henry']
SQL:

assistant: SELECT AVG("Episode") FROM "Table" WHERE "Performer 4" = 'Jimmy Mulville';