00000100000 is still 0.5.
with fixed point, each bit represents 2^n, where n is the bit position in binary (with the MSB being the sign bit with signed numbers)
So with (4 downto -4) = 00100.1010 = 2^2 + 2^-1 + 2^-3 = 4 + 0.5 + 0.125 = 4.625
---------- Post added at 10:21 ---------- Previous post was at 10:18 ----------
Btw, sfixed is just a convenient package for an integer. It is still a 2s complement integer, but the array indeces show the offset of the separation between integer bits and fractional bits.