I guarantee the pagerank will not go away for at least 30 days. The pagerank shouldn't disappear at all for the life of the domain. However, it will also depend on the content you put up on each domain.
But if the pagerank falls below 4 or is lost, I will give you your money back, just push the domains back to me.
You guys are really weird ... selling dubious domains back and forth to each other, over and over -- and for so little profit. Ever hear of www.domaining.com or www.valuate.com?