Math-class, folks

:
4 min = 240 s
1 B = 8 b
1 kB = 1024 B
4 min at 8 kbps = 240 s * 8000 b/s * 1/8 B/b * 1/1024 kB/B= 234.375 kB
4 min at 16 kbps = 234.375 * 2 = 468.75 kB
4 min at 32 kbps = 468,75 * 2 = 937,5 kB
4 min at 48 kbps = 468,75 + 937,5 = 1406.25 kB
Basically, the bitrate alone determines the filesize - samplerate, mono etc are settings to improve the sound at that bitrate, but they are irrelevant for the filesize. If you are encoding "The sound of silence", they should be totally irrelevant, with the exception of support from decoders.
Edit1: "untruncation", Edit2+3: small formula change for better understanding