Finding any information on command line scripting for Fireworks CS4 was difficult. Below is an example script to get you started, along with some hard-to-find resources below. Feel free to add other examples / links to the comments section, so that hopefully this page can become a helpful resource to others.

This script goes across all layers in a Fireworks animated GIF source file (PNG format). It replaces all text layers of a specific name with specific text. I used Ruby to dynamically generate the "process_image( ... )" calls, as a way to iterate through many source files and save animated GIF's with variable text in them.

try
{
    process_image( "hello world" );
    process_image( "konichi wa sekai")
}
catch(e)
{
    alert(e);
}

// this will insert text into every text layer (of every state, if an animated GIF) called "dynamic_text"
function process_image( text )
{
    var dom = fw.openDocument( "foo.png" );
    
    if ( dom == null )
    {
        alert( "Could not open fireworks file: foo.png" );
        return;
    }
     
 if ( !iterate_and_replace_layers( dom, text ) )
 {
     alert( "Did not find a layer to replace in " + source_file + ". Be sure you have the updated source files." );
     return;
 }
 
    var opts = new ExportOptions();
    opts.exportFormat = "GIF animation";
    
    var output = save_directory + source_file_name + '_' + phone_number + '_' + creative_id + '.gif';
    var save_result = fw.exportDocumentAs( dom, output, opts );

    fw.closeDocument( dom, false );
}

function iterate_and_replace_layers( dom, text )
{
    var layer_replaced = false;
    
    for(var i = 0 ; i < dom.frames.length ; i++)
 {
  for(var j = 0 ; j < dom.frames[i].topLayers.length - 1 ; j++)
  {
   var topLayer = dom.frames[i].topLayers[j];
   var elems = topLayer.elemsandsublayers;   
   for(var k = 0; k < elems.length; k++)
   {    
    // add to layer
    if ( elems[k].name == "dynamic_text" )
    {
        set_text( elems[k], number );
        layer_replaced = true;
    }
   }
  }
 }
 
 return layer_replaced;
}


function set_text( element, number )
{
    var runs = element.textRuns;
    var runsArray = new Array();
    runsArray[0] = { changedAttrs: {}, characters: number };
    runs.textRuns = runsArray;
    element.textRuns = runs;    
}

Save the file with a .jsf extension, and to call it from a Ruby command script, simply do:
system( "open script_fw.jsf" )


Article with some information, and a few helpful links to resources:
http://www.adobe.com/devnet/fireworks/articles/demo_current_document_command.html

Link to PDF API documentation:
http://help.adobe.com/en_US/Fireworks/9.0_Extending/fireworks_cs3_extending.pdf

Fireworks Help
http://help.adobe.com/en_US/Fireworks/10.0_Using - (Automating Tasks)

Javscript source of existing Fireworks plugins found here (on a Mac machine using CS4). This is a good way to learn the scripting language by viewing plugins' source code:
~/Applications/Adobe Fireworks CS4/Configuration/commands/