Sure, I am the Backup, I don't see why not. The only possible glitch I can see is that what you can now purchase is Retrospect Mac 15; Retrospect Mac 14 is obsolete. The cumulative Release Notes for Retrospect Mac 15.0 say "New: Support for LTO-8 tape devices", so the chances are the support for LTO4 devices is still there.
In case it isn't, I imagine you can go through Retrospect Sales to get a license code for Retrospect Mac 14.6, which can still be downloaded.
The one thing I would caution you about is that the Desktop Edition only supports a single non-autoloader tape drive. If your LTO4 tape drive is fancy-shmancy, you may have to purchase the Single Workstation 20 Client Edition—which is substantially more expensive. You can probably negotiate this with Retrospect Sales, with whom I have no connection other than as a customer.