  The discovery of superconductivity in a $d^{9-{\delta}}$ nickelate has
inspired disparate theoretical perspectives regarding the essential physics of
this class of materials. A key issue is the magnitude of the magnetic
superexchange, which relates to whether cuprate-like high-temperature nickelate
superconductivity could be realized. We address this question using Ni L-edge
and O K-edge spectroscopy of the reduced trilayer nickelate $d^{9-1/3}$
La4Ni3O8 and associated theoretical modeling. A magnon energy scale of ~80 meV
resulting from a nearest-neighbor magnetic exchange of $J = 69(4)4$ meV is
observed, proving that $d^{9-{\delta}}$ nickelates can host a large
superexchange. This value, along with that of the Ni-O hybridization estimated
from our O K-edge data, implies that trilayer nickelates represent an
intermediate case between the infinite-layer nickelates and the cuprates, and
suggests that they represent a promising route towards higher-temperature
nickelate superconductivity.
