You have already had some excellent guidance from @benwiggy and @dspreadbury. I wanted to add some details on the upgrade process from a user perspective.
When you buy your Dorico 4 Pro upgrade, you get a Download Access Code. When you redeem that code through Steinberg Download Assistant, eLicenser Control Center ‘upgrades’ your existing Dorico to a “not upgradable” licence for the same version, which allows you to continue using eLicenser versions of Dorico, but cannot be upgraded again. After the eLicenser transaction has taken place, you can activate Dorico 4 via Steinberg Licensing. You do not need to have Dorico 3.5 installed to upgrade - merely have the eLicenser licence for 3.5 on your system.
The easiest way ahead is to buy the upgrade and activate it on your existing computer. If Dorico 4 works acceptably, great - you can defer a hardware upgrade. If you want to upgrade to a new computer, you can have both computers activated via Steinberg Licensing. If you stop using Steinberg Licensing software on your old computer, deactivate the licenses to free up the activations.