How do I remove old employers mailbox from my i4 without losing anything else?
If you are talking about the Exchange link: All services bound to that account will be removed. For example, if you add a new contact, the iPhone should have asked you where you want to add them. Either in the Exchange account or in the iCloud. If you've selected Exchange for this all the time, the contacts are saved in the Exchange account and will be gone once you removed the company account.

Included in Exchange can be: Mail, Contacts, Calendar, Reminders and Notes.