If you will be satisfied with an *ad hoc* solution (in other words, you aren't going to make it general enough to handle leap years automatically, etc.), it could go something like:

1. Calculate the principal remaining after the most recent 12/31 payment.

2. Multiply that principal by (1.04)^((day of the year on which you would like to pay it off)/(number of days in the year)) to get the total due.

It appears your payment was calculated by assuming the $32,500 gathered interest from 1/4/2014 to 12/31/2014 using $32,500 * (1.04^(360/365)) = $33,781.85.

That amount can then be used in the Excel PMT function: =PMT(4%,13,-33781.85,0,1) gives $3,252.92. Close enough to $3,252.96?

To calculate the principal remaining after the most recent 12/31 payment, use -CUMPRINC(4%,13,33781.85,1,number of 12/31 payments you have made,1) and subtract that result from $33,781.85.