some tips to reduce your jar size:
-use pngcrush or similar on your graphic file
-rename the graphic file to "m" instead of "m.png"
-convert all the static methods to non-static methods
-try to reduce the number of methods used (each method needs an entry for its signature in the resulting class file)
-I am not sure, but does your webstart version of the JAR selfexecuting? if so it is not necessary so remove the meta-inf directory to save space. (you will need to put it back in for your downloadable JAR version (which can be over 4k as long as the webstart version is under 4k)
-reduce the number of static and class variables as like with the methods, these variables' signatures are recorded in the class file. Instread try to make 'method scope' variables where you can.
Ideally you will only have two 'methods' one main entry method which will call the constructor where all your code is.
The main trick to fit more into 4k is to NOT think OO and do nastry procedual code
The final point is to run your .class through an optimiser (see 4KJO for a tool which attempts to find the best combination)