All $(4,1)$: Sigma Models with $(4,q)$ Off-Shell Supersymmetry
  Off-shell $(4,q)$ supermultiplets in 2-dimensions are constructed for $q=1,2,4$. These are used to construct sigma models whose target spaces are hyperk\"ahler with torsion. The off-shell supersymmetry implies the three complex structures are simultaneously integrable and allows us to construct actions using extended superspace and projective superspace, giving an explicit construction of the target space geometries. 
Chris Hull and Ulf Lindstr\"om
hep-th math-ph math.MP