I don't believe there has been any confirmation on the Phase 4 client being useable at launch.
Removing Phase 3 client, and having a new client for Phase 4 is true however, you can find the answer in the beta forums, and its testing schedule thread.
Here is the link: http://forum.square-enix.com/ARR-Test/threads/77221-Beta-Test-Phase-3-Schedule-%2807-08-2013%29?p=934713#post934713
And below is the post in question
Beta test phase 3 will conclude on Monday, July 15th.
Character Data From Beta Test Phase 3
All character data will be deleted at the end of beta test phase 3. Although character data from Version 1.0 will also be deleted, data from the final save on October 31, 2012 will be transferred again at the beginning of beta test phase 4.
The Beta Test Phase 3 Client
Both the Windows and PS3 clients used for beta test phase 3 cannot be used for beta test phase 4 or official service. Please delete the client at the end of this beta test phase.
Character Appearance Data
Character appearance data saved during beta test phase 3 can be used when creating a character during beta test phase 4 and official service. Those who wish to use their appearance data should not delete the following folder:
My Documents -> My Games -> FINAL FANTASY XIV - A Realm Reborn (Beta Version)
(From "XMB") Game â†’ Saved Data Utility (PS3â„¢)
* Existing character save data cannot be used after phase 3, and can safely be deleted.
Those who do not intend to use character appearance data from beta test phase 3 may delete this folder. However, windows users should please note this folder also contains screenshots. Please move any screenshots you wish to save to a separate location before deleting this folder.
Linking Square Enix Accounts to Sony Entertainment Network Accounts
Square Enix accounts linked to Sony Entertainment Network accounts during beta test phase 3 will be unlinked at the end of testing.
Beta Test Phase 4
Details on the beta test phase 4 schedule, testing scope, and client will be announced at a later time.
Hope this helped.