To set formatting of floating variables you can use a combination of setprecision(n), showpoint and fixed. In order to use parameterized stream manipulators like setprecision(n) you will have to include the iomanip library:
#include <iomanip>
setprecision(n): will constrain the floating-output to n places, and once you set it, it is set until you explicitly unset it for the remainder of the stream output.
fixed: will enforce that all floating-point numbers are output the same way. So if your precision is set to 4 places, 6.2, and 6.20 will both be output as:
6.2000
6.2000
showpoint: will force the decimal portions of a floating-point variable to be displayed, even if it is not explicitly set. For instance, 4 will be output as:
4.0
Using them all together:
outfile << fixed << showpoint;
outfile << setprecision(4);
outfile << x;