OK. So you have a libc4 (a.out) application, presumably without source code.
Getting it to run will be tough! a.out is completely unsupported by now.

To run this application, you would need:
  * a.out support in your kernel;
  * The a.out dynamic linker. There is still one in ld.so-1.9.9, but
    it is not officially supported since 1.9.0, so there may be subtle
    problems. The a.out dynamic linker is /lib/ld.so (libc5 is called
    ld-linux.so.1.x.y, libc6 is called ld-linux.so.2.x.y);
  * All libraries needed by this application, among them the X-libraries.
    You will want a binary distribution of this, because compiling a.out
    libraries takes a lot of courage. Your best bet will probably be
    an old distribution. Try to get an as up-to-date version as possible.

Install ld.so in /lib (it has to be there). Install libc.so.4.x.y and
links in /lib, as some real old a.out applications expect it there.
Install all other libraries in a new directory, for example
/usr/i486-linux-libc4/lib. Add this directory to /etc/ld.so.conf. Run
ldconfig, followed by 'ldconfig -p', to check whether your version
of ldconfig understands about a.out libraries (if not, it will probably
still work, though you might get some warnings every time you run
your a.out application). Now you should be set up.

Note that I have not tried this myself (I am totally uninterested in
a.out compatibility), so any real-life experiences are welcome!

  Frodo