Depends on how many you want to be able to try your work out. To ease
my work, send your entry as a .zip or .tar.gz package that contains a
directory with the same name, suffix stripped. (That is: if your entry
is called gebooie
, send an e-mail to lang@esoteric.sange.fi, to which
you attach gebooie-1.0.tar.gz, which contains a directory gebooie-1.0
where the actual files are). You don't need to use version numbers,
they're just handy if you publish subsequent works after the contest.
Please use a subject line of essie4 entry (category number): gebooie
You should be aware for the consequences of using some esoteric / sparsely available tool. For example, I guess most users will not install SML/NJ on their (Windows?) machines just to try your entry out. Expect amount of commentary to match the ease with which your entry can be tried out.
Note that submitting old stuff is explicitly allowed. Of course, there is little point in doing so if the entry has been submitted to another contest already or if it's well-known enough to have several web sites to discuss it. But if you are going to submit something that has already been published, please don't tell about it before the deadline.
However, everything you submit should be your work, written by you, or at least, contain clear markers of whose work it is, and be submitted will all authors' permission.
The work should be published under a license that is DFSG-free; otherwise, you're causing a lot of excess trouble for other people. You have immaterial rights to your own work anyway. The simplest way to make a piece of work DFSG-free is to place it in the public domain; other options include publishing it under GPL, BSDL, Artistic license or your own DFSG-free license.