You've got 4.0? Send that version to Fractal so they can post it on their web site.
The larger the number, the more recent the firmware. Will the next one be called 3.05 or 4.0? As with any software develoment, that's a judgement call on the part of the developers. If it's a major change, they'll increase the number before the decimal point. (Actually, that's not entirely true; firmware versions that end in .07 involve a shift in the laws of physics.)