If anything, the 4k rule means that you see a lot of games with graphics that are not the most advanced, but some of the best gameplay around. Being forced to focus more on how things work than the quality of your visual media makes for a competition full of innovative and fun ideas.
You'll also see some copycat content, but resist the temptation to call plagiarism. When you think about it, what they're doing is cramming a known title into a 4k file, and getting it to work to the quality of the original. That's incredible, whether you've converted the experience of Super Mario, Football Manager or [partypoker](http://gr.partypoker.com/), and it deserves respect when it feels like you're playing the full game despite the fact it's actually a Java 4k entry.
Last but not least - have fun, even if you're only playing games. If you're making them, well done in advance - it's certainly no easy task to make an addictive, high-quality game with nothing but the smallest of file-space. I'm sure there's a Java 1k competition in the works, or perhaps something even smaller - if not, impress them and enter a 1k title in the 4k category. Good luck, and enjoy the various games you'll play and make over at Java 4k.