Stark spectra have been obtained in the region of absorption by the accessory bacteriochlorophylls (the B-band region around 800 nm) in Rhodobacter sphaeroides reaction centers. The Stark spectra in this region are dominated by the resonance Stark effect of B-L, the accessory bacteriochlorophyll on the functional side; results in Part 1 of this series demonstrated that these effects are associated with the alternative electron-transfer pathway B-L* --> BL+HL- where H-L is the bacteriopheophytin acceptor on the functional side. Low, temperature absorption and Stark spectra for unmodified, Q(A)-reduced and P-oxidized samples of wild-type and three mutant reaction centers (M203GD, M210YF, and M210YW) are presented. This combination samples many perturbations to the BL+HL- state that can be quantified by fitting the data using the theory of resonance Stark effects developed in Parts 2 and 3 of this series. It is found that the mutations perturb not only the energy of BL+HL- relative to that of B-L* but also the electronic coupling between these states and the effective distance of charge transfer for the reaction B-L* --> BL+HL-. Rates for this alternative charge-separation pathway are estimated from spectral analysis and compared with time-resolved measurements reported by several groups. By comparison of the spectra from the Q(A)-reduced and P-oxidized samples, it is found that the dielectric screening is larger in the region around Q(A) and the probe BL+HL- than in the region around P and BL+HL-. The mean frequency of the vibrational modes that are coupled to this charge-transfer process is also estimated. In the context of fitting the Stark spectra, evidence is presented for a strong excitonic interaction between the localized transitions at the B-L and H-L binding sites in reaction centers containing the M214LH mutation and among the localized transitions at the B-L, B-M, and P binding sites in wild-type reaction centers.