Invariant Differential Operators for Non-Compact Lie Groups: the Reduced   SU(4,4) Multiplets
  In the present paper we continue the project of systematic construction of invariant differential operators on the example of the non-compact algebras $su(n,n)$. Earlier were given the main multiplets of indecomposable elementary representations for $n\leq 4$, and the reduced ones for $n=2,3$. Here we give all reduced multiplets containing physically relevant representations including the minimal ones for $n=4$. Due to the recently established parabolic relations the results are valid also for the algebras $sl(8,\mathbb{R})$ and $su^*(8)$ with suitably chosen maximal parabolic subalgebras. 
V.K. Dobrev
hep-th math.RT