  We present far-UV to optical analyses of four hydrogen deficient central
stars of planetary nebulae: BD+303639, NGC 40, NGC 5315 and NGC 6905. Using the
radiative transfer code CMFGEN, we determined new physical parameters and
chemical abundances for these stars. The results were analyzed in the context
of the [WR] => PG 1159 evolution via the transformed radius(Rt)-temperature and
HR diagrams. NGC 5315 showed itself as an odd object among the previously
analyzed central stars. Its temperature (~76kK) is considerably lower than
other early-type [WR] stars (~120-150kK). From our models for NGC 5315 and NGC
6905, it is unclear if early-type [WR] stars have smaller C/He mass ratios than
other spectral classes, as claimed in the literature. A ratio of ~0.8 is found
for NGC 6905. We analyzed FUSE spectra of these stars for the first time, and
identified phosphorus in the spectra of BD+303639, NGC 40 and NGC 5315 through
the transitions P V 1118,1128. The Fe, Si, P, S and Ne abundances were analyzed
in the context of the nucleosynthesis occurring in previous evolutionary
phases. We found evidence for Fe deficiency in BD +30 3639 and NGC 5315, and
from fits to the Si IV lines we determined a solar Si abundance for BD+303639
and NGC 40. For phosphorus, an oversolar abundance in the NGC 5315 model was
preferred, while in the other stars a solar phosphorus abundance cannot be
discarded. Regarding sulfur, we estimated upper limits for its abundance, since
no conspicuous lines can be seen in the observed spectra. We found that Ne is
overabundant in BD +30 3639. In the other stars, Ne is weak or undetectable and
upper limits for its abundance were estimated. Our results are in agreement
with theoretical predictions and show the usefulness of [WR] stars as testbeds
for nucleosynthesis calculations in the AGB and post-AGB phases.(abridged)
