And the order of upgrade should be LDAP servers, Mailstores, then MTAs.
I wouldn't recommend it, but sometimes you
might (note the very tiny 'might') be able to get away with older versions on the MTAs - but just don't waste your time trying/risking different versions between LDAPs & the mailstores. Expecially with 219 bug fixes in 4.5.7 & all the tomcat & ldap changes made. If it fails then your talking
more downtime to restore backups etc.
Are you doing operating system updates at this time as well? - 3 days seems like a bit much for 7 servers.
See the 'Multi-Server Environment Upgrade Steps' at the bottom of the release notes for directions:
/pdf/Zimbra NE Release Notes.pdf