ResolutionMailEnable Standard 1.986 can safely be upgraded to MailEnable Standard 4.x.
MailEnable Standard 4.x should be installed with saving of current server configuration.
Before the upgrade it is recommended to perform backup using MailEnable backup tool.
Additional informationFor information about upgrade to MailEnable 5.x see the article:
11150 Can MailEnable Standard 1.986 shipped with Plesk be upgraded to 5.x version?