Question
When I read a 4-bit (or less) raster dataset, why does FME resample it to 8-bit?
Answer
FME's internal workings don't support anything less than 8-bit, therefore 1, 2 or 4-bit rasters get upgraded to 8-bit on reading.
However, this does not prevent you from reducing the bit size of the data again at the time of writing.