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No lines have been observed for transitions between the doublet and quartetlevels of B I. Consequently, energy levels based on observation for the latterare obtained through extrapolation of wavelengths along the iso-electronicsequence for the 2s2.2p 2Po_{3/2} - 2s2p2 4P_{5/2} transition. In this paper,accurate theoretical excitation energies from a newly developed partitionedcorrelation function interaction (PCFI) method are reported for B I thatinclude both relativistic effects in the Breit-Pauli approximation and a finitemass correction. Results are compared with extrapolated values from observeddata. For B I our estimate of the excitation energy, 28959 +/- 5 cm^{-1}, is inbetter agreement with the values obtained by Edl\'en et al. (1969) than thosereported by Kramida and Ryabtsev (2007). Our method is validated by applyingthe same procedure to the separation of these levels in C II

Toward Calculations with Spectroscopic Accuracy: the 2s2.2p 2Po_{3/2} - 2s2p2 4P_{5/2} Excitation Energy in Boron