We present the results of a deep 300 ks Chandra High Energy Transmission Grating (HETG) observation of the highly variable narrow-line Seyfert Type 1 galaxy NGC 4051. The HETG spectrum reveals 28 significant soft X-ray ionized lines in either emission or absorption; primarily originating from H-like and He-like K-shell transitions of O, Ne, Mg and Si (including higher order lines and strong forbidden emission lines from O vii and Ne ix) plus high-ionization L-shell transitions from Fe xvii to Fe xxii and lower ionization inner-shell lines (e.g. O vi). Modelling the data with xstar requires four distinct ionization zones for the gas, all outflowing with velocities <1000 km s−1. A selection of the strongest emission/absorption lines appear to be resolved with full width at half-maximum (FWHM) of ∼600 km s−1. We also present the results from a quasi-simultaneous 350 ks Suzaku observation of NGC 4051 where the X-ray Imaging Spectrometer (XIS) spectrum reveals strong evidence for blueshifted absorption lines at ∼6.8 and ∼7.1 keV, consistent with previous findings. Modelling with xstar suggests that this is the signature of a highly ionized, high-velocity outflow (log ξ= 4.1+0.2−0.1; vout∼−0.02c) which potentially may have a significant effect on the host galaxy environment via feedback. Finally, we also simultaneously model the broad-band 2008 XIS+HXD (Hard X-ray Detector) Suzaku data with archival Suzaku data from 2005 when the source was observed to have entered an extended period of low flux in an attempt to analyse the cause of the long-term spectral variability. We find that we can account for this by allowing for large variations in the normalization of the intrinsic power-law component which may be interpreted as being due to significant changes in the covering fraction of a Compton-thick partial-coverer obscuring the central continuum emission.