Sun is discontinuing SCJP 1.4 this month. You probably cannot even buy a voucher for SCJP 1.4 anymore.
There's a lot of new stuff in Java 5, so there are lots of new things in SCJP 5 compared to SCJP 1.4. The difference between SCJP 5 and SCJP 6 is very small. The differences are described in more detail in the JavaRanch
SCJP FAQ.
So, I'd recommend you to study for SCJP 6.