why retire 1.4???...when you have been using it for 4 years and you just need the lame SCJP in order to take a more "important" certification like the SCWCD, for example. If anyone is already proficient with 1.4 just take it.
For most people I would recommend 1.6. It's almost identical to 1.5 and it's the "latest and greatest".
I suppose 1.4 is okay, but frankly Java 5 added SO MANY new features that I strongly recommend skipping the 1.4 and going for 1.5 or again, preferably 1.6.
Spot false dilemmas now, ask me how!
(If you're not on the edge, you're taking up too much room.)