If I add a paragraph to the body of my home page (it's an store) can that alone lower the PR4 it now has?
No. PR is based on number of sites linking to you not the content/body text. You could alter your serps for good or bad by changing content though and that is much more important.
Changing content on your front page is a very good idea. What not to do is change your Title tags or other Meta. That will get you boxed by G. D
In my opinion Title is like BRAND NAME of html page and it better do not change frequently, if you have good SERP in google or MSN. We can use different BRAND NAME as per theme of different pages of site
PR got nothing to do with content. And it's better to update your page copy since I think SE love site updates.