Your output is a 2x4 matrix. Using Assignment block, you could either
Provide a scalar value in U, assign multiple elements of the 2x4 matrix
Or, Provide the same size (2x4) value in U, assign seletive elements of the 2x4 matrix.
I couldn't figure out a way to do what you want, which is reshape plus shuffle.
The easiest approach is to use a MATLAB Function block. It just needs one line of code.