When you try to select the T4 Primary Box Number on the Deduction Code Setup, it will not accept Box 84. Box 84 is used to track Public Transit Passes.
Unfortunately Box 84 is not currently available on the T4. There is currently no way to store this information in the CPY table structure.
Update the T4 manually for Box 84.
Please access MS Connect and vote on the product suggestion if you would like to see this functionality considered for a future enhancement:
Article ID: 2545803 - Last Review: Feb 25, 2016 - Revision: 1
Microsoft Dynamics GP 2015, Microsoft Dynamics GP 2013, Microsoft Dynamics GP 2010, Microsoft Dynamics GP 10.0, Microsoft Dynamics GP 9.0, Payroll - Canada