You can subscribe to the Photography plan, which includes Lightroom Classic CC, also known as Lightroom 8. Make sure your operating system can run this software Lightroom Classic CC system requirements
As dj_paige wrote you can subscribe to the Photography plan.
Another point is to buy an update to Lightroom 6 here:
But you have to know that LR 6 is the latest version of LR with perpetual license and there are no more updates and patches.
In what respect.
Upgrade to a newer version for a lower fee than the full license?
Or UpDate to the newest Point Release version of LR 4, which is 4.4.1?
The only Upgrade for a one time fee is LR 6 which is still available AFAIK. But that version is 2 major releases behind and will not get any new updates of any kind.
Other than that you have to Subscribe to the Photographer plan for a monthly fee.