Hello galeba,
Thanks very much for the interest in upgrading.
Once you've purchased the v5 upgrade from the shop, the only deadline for actually using it is that you might want to have installed it and used its 'File | Initiate migration of Hauptwerk pre-v5 sample set licenses to this version ...' menu function to migrate your v4 licences to v5 by the 1st October 2020, in order to be certain that there won't be any additional charge for migrating the sample set licences: viewtopic.php?f=10&t=17833#p133611
(For example, you might possibly decide you'd prefer to wait before installing it until v5.0.1 is available, e.g. if any of the v5.0.0 known issues and bugs would be of relevance to you: viewtopic.php?f=10&t=17970 .)