Nikon D4S - Manual (Page 58)

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Press the I (Q) and O (Q) buttons again.
Press the I (Q) and O (Q) buttons together a second time
while C is flashing to format the memory card. Do not
remove the memory card or remove or disconnect the power
source during formatting.
When formatting is complete, the top
control panel will show the number of
photographs that can be recorded at
current settings and the frame-count
displays will show
B.