  We present a comprehensive examination of the X-ray variability of the narrow
line Seyfert 1 (NLS1) galaxy NGC4051. We combine over 6.5 years of frequent
monitoring observations by RXTE with a >100ks continuous observation by
XMM-Newton and so present a powerspectral density (PSD) covering an
unprecedented frequency range of over 6.5 decades from <1e-8 to >1e-2 Hz. The
combined RXTE and XMM-Newton PSD is a very good match to the PSD of the
galactic black hole binary system (GBH) Cyg X-1 when in a `high', rather than
`low', state providing the first definite confirmation of an AGN in a `high'
state. We find a break in the PSD at a frequency \nu_{B}=8e-4 Hz. If \nu_{B}
scales linearly with mass then, assuming a black hole (BH) mass of 10 M_solar
for Cyg X-1, we imply a BH mass of 3e-5 M_solar in NGC4051, which is consistent
with the recently reported reverberation mapped value. Hence NGC4051 is
emitting at \sim30% L_{Edd}. We note that the higher energy photons lag the
lower energy ones and that the lag is greater for variations of longer Fourier
period and increases with the energy separation of the bands. Variations in
different wavebands are very coherent at long Fourier periods but the coherence
decreases at shorter periods and as the energy separation between bands
increases. This behaviour is similar to that of GBHs and suggests a radial
distribution of frequencies and photon energies with higher energies and higher
frequencies being associated with smaller radii [ABRIDGED].
