Your best bet is to purchase a used copy from Amazon eBay.
However, it would be much wiser to go to a newer version of the program. For one thing, version 4 is only capable of editing video from tape-based miniDV and HDV camcorders captured over a FireWire connection -- all of which hardware is no longer being produced.
Also -- and this is a big deal -- a number of the components in version 4 will not activate because they are so old that the web sites that activated them no longer exist.
In other words, there are a number of liabilities to installing version 4 and virtually no advantages. I highly recommend you consider a newer version.