Your pill "Last 4 Weeks % Chg YoY" is a nested table calculation, so when you go into the pills, Edit Table Calculation dialog, you get an additional drop-down to select the field. You want "Last 4 Weeks" set to use "Week" and "Last 4 Weeks % Chg YoY" to use "Year".
This works really well, and you made it look so easy.
Actually, believe it or not, I submitted this same question to Tableau support and they recommended doing the calculation in Excel (Tableau case 00066537)
It has been my impression that if you are looking for how to help, the forum is a great resource, and Tableau Support is the best resource for when Tableau errors. I think this is why Tableau has Professional Services in addition to Support. The forum is great for when you want to do-it-yourself, but don't know exactly how to get the results that you want.