As long as the previous sites are using PHP, eCart 5 will work without a problem.
If you have ASP or ColdFusion sites, you will need to use eCart to maintain them. If this is the case, you should create a secondary computer user, give this user administrative privileges, and install eCart 4 under this user. you will also need to recreate the site definitions in Dreamweaver under the second user. When you need to modify one of these site, log out of the primary user, and into the secondary user to use eCart 4.