You can continue using Revoice Pro 4 after you upgrade to Revoice Pro 5.
When you upgrade, you will surrender your existing license for Revoice Pro 4 and you will receive a group license that will let you use Revoice Pro 4 and Revoice Pro 5 on up to two computers at the same time.
A group license is locked, so the Revoice Pro 4 and Revoice Pro 5 license will be activated together on the same computer; however, the group license can be active on up to two different locations at the same time, so you still get a lot of flexibility.
This iLok video illustrates the surrender process you will follow to upgrade:
More information on using a Synchro Arts iLok license on two computers at the same time is provided in this article: Can I use a Synchro Arts iLok license on two computers at the same time?