Repeated imaging observations have been made of NGC 4639 with the Hubble Space Telescope between 1995 April and July over an interval of 71 days. Images were obtained on 12 epochs in the F555W band and on two epochs in the F814W band. The galaxy is parent to the prototypical Type Ia supernova SN 1990N. A total of 18 definite and six likely Cepheids were found with periods ranging between 17 and 67 days. The dereddened distance modulus of NGC 4639 derived from the Cepheids that could be measured in both passbands is (m - M)0 = 32.03 +/- 0.22, which corresponds to a distance of 25.5 +/- 2.5 Mpc. The apparent magnitudes of SN 1990N at maximum of Bmax = 12.70 +/- 0.05 and Vmax = 12.61 +/- 0.05 combined with the Cepheid distance modulus gives MB(max) = -19.33 +/- 0.23 and MV(max) = -19.42 +/- 0.23 for SN 1990N. Combining these with data for five other SNe Ia calibrated in earlier papers of this series and with a preliminary calibration of SN 1989B gives <MB(max)> = -19.52 +/- 0.07, and <MV(max)> = -19.48 +/- 0.07. Combining these mean absolute magnitudes with the Hubble diagrams for well-observed blue SNe Ia in B and V read at remote redshifts well beyond all local velocity anomalies gives a mean Hubble constant ofH0=58+7-8 km s-1 Mpc-1(external error) using no second-parameter corrections. Analysis is also given using various formulations of second-order corrections depending on Hubble type of the parent galaxy, color of the SNe Ia, and/or light-curve shape, which show that these uncertain corrections affect the value of H0 by less than 10%.Finally, we argue that the short distance scale espoused from both the distance to M100 in the Virgo Cluster and NGC 1365 in the Fornax cluster (Freedman et al.), giving H0 = 80 +/- 17 and H0 ~ 70, respectively, have systematic errors in their precepts concerning the distances of the cluster cores, which when corrected give the long scale with H0 ~ 55.