The A3640 will make your A4000 faster, but without it's own fastram IMO speedup won't be that big.
Adding an Xcalibur would be speeding it up a lot more, but they are really a rare find and you would have to be prepared to pay big bucks.
See here for more info on the XCalibur.
http://www.bboah.com/index.php?action=artikel&cat=45&id=2094&artlang=en IMHO it would be better to save your money for a Cyberstorm 040 or 060. (edit: or any other accelerator which also adds fastram).
