Personally I would never buy flash 4.0 anymore it's outdated in my opinion.
Flash MX comparing to flash 5 has some new features, and has some new codinginterpetations. So terms from flash 4.0 / 5.0 have become 'depreciated' and have been replaced by new codings. Which can be seen as 'more logical', 'less CPU usage' etc.
Flash MX 2004 (and recently flash 8) uses actionscript 2.0 which is quite different if you're used to flash MX. But again, different functions, better ones, new ones.
Remember that you can always download at Macromedia's a trial version. So can play around and see if this ones meets you're expectations.