  We present an analysis of the high frequency X-ray variability of NGC 4051
(M_BH ~ 1.7*10^6 M_sun) based on a series of XMM-Newton observations taken in
2009 with a total exposure of ~570 ks (EPIC pn). These data reveal the form of
the power spectrum over frequencies from 10^-4 Hz, below the previously
detected power spectral break, to > 10^-2 Hz, above the frequency of the
innermost stable circular orbit (ISCO) around the black hole (nu_ISCO ~ 10^-3 -
10^-2 Hz, depending on the black hole spin parameter j). This is equivalent to
probing frequencies >1 kHz in a stellar mass (M_BH ~ 10 Msun) black hole binary
system. The power spectrum is a featureless power law over the region of the
expected ISCO frequency, suggesting no strong enhancement or change in the
variability at the fastest orbital period in the system. Despite the huge
amplitude of the flux variations between the observations (peak-to-peak factor
of >50) the power spectrum appears to be stationary in shape, and varies in
amplitude at all observed frequencies following the previously established
linear rms-flux relation. The rms-flux relation is offset in flux by a small
and energy-dependent amount. The simplest interpretation of the offset is in
terms of a very soft spectral component that is practically constant (compared
to the primary source of variability). One possible origin for this emission is
a circumnuclear shock energised by a radiatively driven outflow from the
central regions, and emitting via inverse-Compton scattering of the central
engine's optical-UV continuum (as inferred from a separate analysis of the
energy spectrum). A comparison with the power spectrum of a long XMM-Newton
observation taken in 2001 gives only weak evidence for non-stationarity in
power spectral shape or amplitude. [Abridged]
