  We present an experimental study of seeded four-wave mixing (4WM) using a
diamond excitation scheme (with states from the 5S$_{1/2}$, 5P$_{1/2}$,
5P$_{3/2}$ and 5D$_{3/2}$ terms) in a thermal vapour of $^{87}$Rb atoms. We
investigate the 4WM spectra under the application of a strong magnetic field
(0.6 T). The Zeeman interaction is strong enough to realise the hyperfine
Paschen-Back regime, which has the effect of separating the optical transitions
by more than the Doppler width, thereby significantly simplifying the spectral
features. We show that this facilitates a quantitative comparison, even in the
regime of strong dressing, between experimental data and a simple theoretical
model based only on four-level optical Bloch equations.
