We get asked this question all the time here at http://www.visitcairns.com.au and the answer is pretty simple.

If you have time/money do the 5 day course.

It's like doing a dive course and dive trip in one.

With a 4 day course you'll do 4 dives, which is the minimum requirement to qualify for your dive certificate. They will all be "training dives".

A 5 day course will usually give you 9 dives. Now the first 4 are training dives but for the next 5 dives you are a certified diver. One of these usually is also a night dive. Another advantage is that the dives will be with the other students from your course who have the same skill levels as you.

So the first dive trip you do after completing your course you will have already experienced 5 solo dives and be that little bit more skilled and confident.

Also the price per dive is about the lowest you'll ever find.