average first 4 months - measure - power pivot

Attached is a sales analysis I am working on. On the work sheet called "sales_by_customer" you will find sales revenues for a store with customer refs in the rows and the applicable months in the columns.

I would like to add an additional column using a measure to calculate the four month average revenues for each customer for the months of Jan, Feb, March and April.

Could someone please help me write this measure.

Note: My pivot table is linked to a data model in power pivot so it must be a measure and not a traditional calc field which could be done using excels native features.

Thanks
Mike
trans.xlsx