The problem come down to the fact the the way scripts are recorded in actions the full path of the script is recorded. If you un-install CS3 its script directory will be deleted so the script is gone CS4 script is in a different directory.

CS2 and prior recording of the plug-in fit image were recorded as step Fit Image not as a script path step. These actions should still work for when Photoshop starts up the CS4 Fit Image scripts register as the plug-in Automate>Fit Image. When these action are played and the plug-in step Fit Image get played it is looking for the registered Fit Image Plug-in Photoshop should run the CS4 Fit Image Script.

The bug in Fit Image is very minor. One day I used fit image on a image that was 1080 x 1080 and fit image was set to 1200 x 1200 the resulting image turned out to be 1201 x 1200 one side was one pixel off there were 1200 extra pixels.

Reading the CS3 Fit Image Script I saw this:
docRatio = docWidth / docHeight; // decimal ratio of original width/height

// NOTE - ccox - 17 Aug 2004 - I added the rounding by 0.5 // this should solve reported cases of fit image being off by 1 (always under) // NOTE - elr - 3 May 2006 - keep original aspect ratio
newWidth = width;
newHeight = ((1.0 * width) / docRatio) + 0.5; // decimal calc newHeight = 1 * newHeight; // make integer

if (newHeight > height) {
newWidth = 0.5 + docRatio * height; // decimal calc
newWidth = 1 * newWidth; // make integer
newHeight = height;
}

Someone fixed fit image not to be one pixel under. I thought this fix most likely cause my one over problem so I stuck in some alerts to see the numbers. Looked like the multiply by one to round did not round and I thought the .5 value perhaps would work better if it were .4. So I changed to code to

docRatio = docWidth / docHeight; // decimal ratio of original width/height

// NOTE - ccox - 17 Aug 2004 - I added the rounding by 0.5 // this should solve reported cases of fit image being off by 1 (always under) // rounding by .5 seem to make some sides 1 Px large change to .4 and used math round to round JJMack
// NOTE - elr - 3 May 2006 - keep original aspect ratio
newWidth = width;
newHeight = ((1.0 * width) / docRatio) + 0.4; // decimal calc newHeight = Math.round(newHeight); // make integer

if (newHeight > height) {
newWidth = 0.4 + docRatio * height; // decimal calc
newWidth = Math.round(newWidth); // make integer
newHeight = height;
}

The name of my script file is "My Fit Image.jsx" I renamed Adobe file to "Fit Image.jsx.Adobe" My script is in My Photoshop Script tree which I linked to Photoshop's Presets\Script\ by placing a shortcut to my tree into into Photoshop's Script directory. "My Fit Image.jsx" registers itself as Automate>Fit Image...

<name>$$$/JavaScripts/FitImage/Name=Fit Image...</name> <about>$$$/JavaScripts/FitImage/About=Fit Image Version 10.0 By Quality Process^r^rCopyright 2007 Adobe Systems Incorporated. All rights reserved.^r^rResizes a document constrained by the given bounds.^r^rJohn J. McAssey Modified Version^rFixed the one side one pixel to long problem^rand changed the interpolation Method used^rUpsizing will use Bicubic Smoother^rDownsizing Bicubic Sharper^rInstead of just using Bicubic for all resizing like Adobe did.</about>
automate

jjmack

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CS4: Fit Image Problem