Had the same thing happen to one of my sites - domainofmusic.com
Went from 5-4, with all sub pages at 4.
I wouldn't really worry at all - if you had gone from
a 6 to a 5, I would be seriously worried - there's a relatively
small difference from a 4 to a 5 (4 is 3790, 5 is 22680)
but a huge leap from 5 to 6 (22680 to 136500 -
PR is exponential).
What did I do? I took my site out of frames, optimized
as many of the main pages as I possibly could,
updated all of my keywords and meta info, interlinked
all of the main pages, and submitted my site to as
many of the top 20 sites under 'classical music' as
I could. Now I just have to wait, but it shouldn't
take much longer than a few months
And yes, in answer to your question - do expect fluctuations from time to time, but the best tip I can give you, is to update your content and your site overall as much as you possibly can. Google loves fresh,
relevant content, but WILL (notice the absense of the
word 'may') lower the visibility of static sites.
This is what happened with Domainofmusic.com - I hadn't done anything with the site in 3 months, and
if I actually made any money off of that project, it would be seriously bad for business.