Num(Left(:Column, 4), << Use Locale(0))
Also, in general what are is your decimal separator? If it is comma, it could be causing the issue. You can also first try to "solve" this in JMP Script Editor to avoid all possible "issues" JMP table could be causing
Show(Num(Left("0.01 - 0.02", 4), << Use Locale(0)));
Show(Num(Left("0,01 - 0,02", 4), << Use Locale(0)));
Show(Num(Left("0.01 - 0.02", 4), << Use Locale(1)));
Show(Num(Left("0,01 - 0,02", 4), << Use Locale(1)));
Num() will return missing value if it isn't able to convert the string to a number.

-Jarmo