This paper compares the performance of the Prochem Maxflo T and Lightnin A315 hydrofoil impellers, a Rushton disc turbine, and a mixed flow impeller in water, electrolyte solution, and viscous shear thinning sodium carboxymethylcellulose solution. Impeller performance was compared on the basis of(a) the reduction in power drawn on aeration, for operation at equal ungassed specific power inputs, (b) the power required to disperse gas at a given gas flow rate, (c) the gas holdup generated at a given gassed specific power input and superficial gas velocity, and (d) flow stability. The equipment, techniques, and model fluids used are described in parts 2 and 3 of this series of papers. Hydrofoil impellers can maintain a higher relative power than either the Rushton turbine or mixed flow impeller over a wide range of gas flow rates, and the overall fall in power drawn is much less (typically similar to 30-50%, as compared with 50-65%). This is advantageous in terms of operating efficiency and implies a higher gas-liquid mass transfer potential since the latter is directly related to the gassed specific power input. Both classes of downward pumping impeller are prone to flow and associated torque fluctuations, while operation with the Rushton turbine is much more stable. The A315 is more energy efficient in dispersing gas than the Prochem. Although in this respect the efficiencies of the A315 and Rushton turbine are similar in water, the latter is superior at low and moderate gas flow rates in CMC solution. Gas holdup can be correlated with gassed specific power input and superficial gas velocity, and the holdups produced by the different impeller types in a given model fluid are broadly similar.