FV=P{[1 + R]^N - 1/ R}
87,000 = P x {[1 + 0.04/12]^(13*12) - 1 / [.04/12]
87,000 = P x [ 0.68057375.... / 0.0033333...]
87,000 = P x 204.172125...
PMT = 87,000 / 204.172125
PMT =$426.11 - monthly deposit required to save $87,000 over 13 years @ 4% comp. monthly.