This reminds me of a joke about bread and eggs. The "if" is not dependent on the 4+, you always get a new power if the slain model was a Psyker. If it was dependent on the 4+, it would have to say "On a 4+, Yvraine regains one lost wound [AND] If the model was a Psyker you can generate one additional Psychic Power from the Revenant Discipline for Yvraine." It does not say that, so you get the power even if you fail the 4+.
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