OH? The game must be able to play on a Mac as well? Oh man... that spoils part of my programming...
Every year there are one or two games that fail to work on one or other of the judges machines resulting in a poor score. So it pays to publish a beta version during the competiton & ask people to confirm in the thread comments, which OS they are using.
It's never been entirely clear whether the 4K games have to be cross-platform. There have been suggestions that the games only have to work on Windows, but in previous years, the occasional judge controversally has had a Mac or Linux machine, so I try to play safe and check cross-platform as much as possible. Even if one only targets a single OS (e.g. Windows) there are gotcha's across OS versions. Timer granularity on Windows 95,98,Me is about 50ms or so (64ms maybe), while Windows XP is around 16ms (or so) in the 1.4.2 JRE. This can result in jerky movement on old machines. On OS X, there are an increasing number of rendering bugs with older OS. All Macs are fussy about the choice of BufferedImage format, particularly if you use Alpha channels. Go with a pre-multiplied ARGB_PRE format to avoid problems. Last year I spent considerable time getting my entries to run on OS X 10.3, which was difficult as I don't have that OS version. Linux is almost impossible to test for, since there are so many variations. However it is well known that Full Screen applications often fail on Linux, so it is best to go for Windowed mode, or at least start windowed and provide a key to select Full Screen. It also pays to check that your App works on a minimal spec machine (say 1.5GHz), since not everyone can afford a new machine. I generally just add a big delay in my main loop, to simulate this during testing.
So, the moral of the tale is test with Windows XP first, and then ask friends and forum members to test other OS versions during the competition period.
Of course, if you are running your own closed competition for your educational institution, you can set whatever rules you like