The effective interest rate is calculated through the equation,
ieff = (1 + i/r)^r - 1
where ieff is the effective interest, i is the nominal interest, and r is the number of 15 weeks in a year. Every year, there are 52 weeks. Thus, there are 3.467 15-weeks approximately. Substituting this into the equation,
ieff = (1 + 0.04/3.467)^3.467 - 1
ieff = 0.04057
ieff = 4.057%
