[Andrew9999]

As a software publisher myself I've always found the most successful approach has been to offer buyers who recently bought v4 a free upgrade to v5 (so they don't feel cheated or put off buying waiting for v5), and then simply charge everyone else a flat upgrade fee for v5. I wouldn't make the upgrade fee too small either - in effect this is your software maintenance charge, and that can typically amount to 20% or more of purchase price per year for most software... admittedly I'm more used to B2B than B2C software selling, but this works for us.