Well, a whole list of
new features was added in Java 5 - generics, annotations, autoboxing, enums, etc. - which you'll have to learn when you do SCJP 5. On the other hand, a number of topics have been dropped from the SCJP exam (such as bit shift operators).
But really, in my opinion it indeed doesn't make much sense to do SCJP 1.4 instead of SCJP 5 if you're just starting right now.
In the real world, there are still a lot of companies who are still using Java 1.4.2, but more and more are switching to Java 5 or 6, so SCJP 1.4 is becoming more and more obsolete.