DFT frequency resolution exercise
Carry the units and it becomes understandable.

T =  0.03125 seconds per sample

Thus your sampling rate is

1/T = 32 samples per second

Each bin corresponds to the cycles per frame.  Your frame has four samples.  Suppose the bin index is k.

k (cycles per frame) * 32 (samples per second) / 4 (samples per frame) = 8k cycles per second = 8k Hz

So the frequencies for your bins, in Hz, become


k   f_k
0   0
1   8 
2   16
3   24


However, the DFT wraps around, and choice of the principal frequency among many aliases is a matter of convention.  It is mathematically sounder to consider the upper half of the DFT as negative frequencies.  For real valued signals, the upper half is just the complex conjugate of the lower half and thus provides no new information so it is usually ignored.  So, if you want to consider f_3 as 24 or -8, it doesn't really matter.  For a real valued signal, a negative frequency is the same as a positive frequency.