I have a workbook with a summary tab and one tab for ever week.

I need to write a formula that looks for a value from column a, find the
sheet with a matching value in cell e4 and returns the value of cell ac1 of
the worksheet.

This formula will live in column b of the summary sheet.

Example:

Summary Tab

A B
1 19-Jul 19,000(formula result)
2 16-Jul
3 23-Jul
4 30-Jul

Data Tab 1

E AC
1 9-Jul 19,000

It will need to look in cell E1 of every tab in the work book (52 tabs) for
the value found in $a1.