Hi, how do I use the activation code for 3.5 in 4.0?
That sounds like an oxymoron. You have to purchase a new code for Dorico 4. If you purchased 3.5 with an included upgrade, then you should have been sent a new code by e-mail.
Be aware that some Steinberg e-mails (saw this again with the new Cubase upgrade) tend to confuse Access Codes (to be typed into the Download Assistant) with Activation Codes (to be typed into the eLicenser). The Access Codes have fewer digits.
I fell for Dorico 4 and now with Cubase 12, inserting the code in the helicenser control center (now a vice acquired over the years 
) … but finally in some time we will not get confused anymore …
as soon as there are updates for the instruments I have on the dongle I will be able to say goodbye