However you want to obtain YNAB 4, it's going to be a trial but it's really simple to extend that trial by however long you want. To my knowledge there's no difference between the trial and the paid version.
Here I'm talking about YNAB 4.3.857 but this trial date editing probably works on any version of 4.
Install on your computer. Launch YNAB4 for the first time. Click Start Trial.
Close YNAB
Using notepad, open the file %APPDATA%\com.ynab.YNAB4.LiveCaptive\Local Store.lic
Change the <expirationDate></expirationDate> date to be 10 years in the future
Re-open YNAB4. Go to Help->Purchase or Activate YNAB