In Java 5.0, a number of new language features were added (for example: generics, auto-boxing, a new syntax for the 'for' loop, etc.). You'll have to learn those new things if you're going to do the SCJP 1.5 exam.
In my opinion it is best to do the newest available exam (1.5), because it will become obsolete less quickly, and so is more valuable, than an older version.
If you are going to work with application servers, you will find that Java 1.4 is in most cases the maximum version supported. However, knowing Java 5.0 doesn't hurt, in most cases you will just have to be more verbose (no autoboxing so box yourself, no enhanced for loop so use iterators, no generics so implement all checks to guarantee consistancy, etc.).
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