  We rewrite the recently proposed differential expansion formula for HOMFLY
polynomials of the knot $4_1$ in arbitrary rectangular representation $R=[r^s]$
as a sum over all Young sub-diagrams $\lambda$ of $R$ with extraordinary simple
coefficients $D_{\lambda^{tr}}(r)\cdot D_\lambda(s)$ in front of the
$Z$-factors. Somewhat miraculously, these coefficients are made from quantum
dimensions of symmetric representations of the groups $SL(r)$ and $SL(s)$ and
restrict summation to diagrams with no more than $s$ rows and $r$ columns. They
possess a natural $\beta$-deformation to Macdonald dimensions and produces
positive Laurent polynomials, which can be considered as plausible candidates
for the role of the rectangular superpolynomials. Both polynomiality and
positivity are non-evident properties of arising expressions, still they are
true. This extends the previous suggestions for symmetric and antisymmetric
representations (when $s=1$ or $r=1$ respectively) to arbitrary rectangular
representations. As usual for differential expansion, there are additional
gradings. In the only example, available for comparison -- that of the trefoil
knot $3_1$, to which our results for $4_1$ are straightforwardly extended, --
one of them reproduces the "fourth grading" for hyperpolynomials. Factorization
properties are nicely preserved even in the 5-graded case.
