I am a user of Illustrator CS4 and I have been sent a CS5 file. Is there any online converter tool that I can use to downsave the file in CS4 in order to work with it?
No, there isn't. Best method would be to ask for a CS4 file. If that's not possbile you could create a new file and place the CS5 file. This only works if it was saved with PDF compatibility and it will flatten all "live" content.
You could download the trial of CS5, open save as... and save as CS4. (or, maybe it would be the trial of CS6 at this point.)