  The normal state in the hole underdoped copper oxide superconductors has
proven to be a source of mystery for decades. The measurement of a small Fermi
surface by quantum oscillations on suppression of superconductivity by high
applied magnetic fields, together with complementary spectroscopic measurements
in the hole underdoped copper oxide superconductors, point to a nodal electron
pocket from charge order in YBa2Cu3O6+x. Here we report quantum oscillation
measurements in the closely related stoichiometric material YBa2Cu4O8, which
reveal similar Fermi surface properties to YBa2Cu3O6+x, despite an absence of
charge order signatures in the same spectroscopic techniques such as x-ray
diffraction that revealed signatures of charge order in YBa2Cu3O6+x. Fermi
surface reconstruction in YBa2Cu4O8 is suggested to occur from magnetic field
enhancement of charge order that is rendered fragile in zero magnetic fields
because of its potential unconventional symmetry, and/or its occurrence as a
subsidiary to more robust underlying electronic correlations.
