Here's a fully compiled version, principles applied in modifying the code has been mentioned in the links above:
ssamp2 = Compile[{{n, _Integer}, {nd, _Integer}},
Function[xyzw, {{xyzw[[1]] + I xyzw[[2]], -xyzw[[3]] + I xyzw[[4]]}, {xyzw[[3]] +
I xyzw[[4]], xyzw[[1]] - I xyzw[[2]]}}]@#/Sqrt[Total[#^2]] & /@
Flatten[Transpose@RandomVariate[NormalDistribution[0, 1], {n, nd, 4}], 1]]