If you have already bought version 4, just download the new version from the MycoKey website. If you don't yet own MycoKey or if you have a earlier version, download the 14 days trial and check if everything is working. Remember to try to open the free Amanita species key, to check if the pdf-files opens as expected on your system. If satisfied, you may then pay the license and get access to everything.
MycoKey 4.1 is mainly a stability and compatibility upgrade. There are, however, some enhancements to the character coding and also a few new pictures, e.g. of Hirticlavula, Cephaloscypha, Pseudolasiobolus and Torrubiella.