Hello,
Not a good idea.
You could always open your PP4 document within version 5, gradually bring them to version 5 and still have your version 4 intact.
I would also refer you to:
http://www.objectiflune.com/support/faq.aspx Have a look at "Miscellaneous questions"
-What do I need to know before upgrading to PlanetPress Suite Version 5.
There is a document available that explains and said why:
"PlanetPress Watch 4 & 5 cannot both run on the same PC.