My DD's prevenar were longer than 4 weeks, and although it may not be ideal, it is still fine, but as kawazuki suggests some doses may not be needed depending on how much time has passed.
A few weeks will be fine though. There is an immunisation handbook printed by the government that has all the recommendations in it, as well as catch up schedules. I borrowed a copy from my MCHN, you might find it useful.
Here is some more info, it says doses should be 4-8 weeks apart
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