Its all to do with pr flow within the site. With 30 pages you were probably feeding lots of pr back to the home page. Now each page pr is being shared amoungst more links on each page to your new pages. In other words, where you had only a few links per page (each taking a proportion of the pr off the page) now you have more links and thus each link is carrying less pr.

Have you added a lot more links off your home page? If so, it is no surprise that they now make the next level only pr3. The home page may have reduced in pr because now it has pr3 pages pointing back to it rather than pr4.

If you had linked all the new pages from one of your original pr 4 internal pages and left the old pages as they were, I suspect your old pr4 and pr5 home page would have stayed the same.

You should consider how much pr each page needs in order to rank well for its target phrase. The new pages may be targeted at non competitive phrases so they may only need pr1 or 2. If that is the case, you need to redesign the pr flow so that the index page returns to pr5, your next competitve pages pr4 and the non competitve pages can drop to pr1 or2.