Gerber File Format 4.5 or 4.6
This basically controls the number of digits before and after the decimal point in the Gerber output (if you're not omitting leading or trailing zeroes):

4:4 -> 0034.5678

So basically, 4:4 will give you 0.1um resolution, 4:3 will give your 1um resolution, ...

The 4:4 or 4:5 or any other 4:x format is used when you output your Gerbers in [mm]. If you're using imperial numbers, you will typically have something like 2:x.