Couple of things come to mind.
1. If they are both on the same server / same IP then close the O
pr as the cross links to each would be ignored pretty much as they need to always have their own IP address. ( like it or not it's part of the
SEO Pie )
2. Taking the content from O to the
PR 4 should actually help as long as you don't add it all at once. It would be flag as odd to have so many new post etc... just move over 25 or so a week and let the old domain name expire once you have them all moved.
3. Business is always a risk but in this case I'd move forward with the switch.
4. Place Google ad's in the PR4 blog and make them none intrusive maybe it will help pay the rent!
Good luck!