101.6 is the mathematical rounded up equivalent of 4.0 in. 4.0 divided by .0393701 =101.59994.
.0393701 is one of magic numbers (if you will) to convert metric to inches. Used in it's long form, it is quite accurate. it usually gets rounded up to .04, as an example a 7mm- Remington is .280 cal in inches (.04x7=.280) (.0393701x7=.2755907)
So a 4.0 bolt pattern would be 1.6 mm wider than a 100mm bolt pattern. which will put a little stress on the studs.
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