It may depend on your exact requirements, but one way to approach this is to make sure the numbers you have are stored by OpenRefine as a number - they would display as green in the UI and a Numeric Facet on the column would successfully display the data.
If you currently have the information as Strings rather than Numbers you can do a transformation like:
Once you have the data as numbers, then you can covert back to a string with a particular number of decimal places (with rounding) using a transformation like:
value.toString("%.4f")
This would round to 4 decimal places. Change the '4' in this transformation to another number to round to a different number of decimal places
Owen