Upgrading to >= 4.0.0
From this article you can find a checklist what to take into account when you upgrade to Nosto module version >= 4.0.0.
It's recommended not to upgrade any other modules at the same time when upgrading Nosto module to version >= 4.0.0. If other module upgrades alter the product data it might lead into a full re-index of Nosto product data during the module upgrade process. With large catalog and multiple store views the full re-index might take hours to complete.
As with any Magento 2 module upgrade remember to run
bin/magento setup:di:compilecommands. Note that these might be automated by your deployment flow.
If you are upgrading from module version < 3.9.0 you must re-connect your Nosto account. This is due to the fact that order confirmations are also sent through Graphql API that requires an additional authentication token.
As version 4.0.0 introduces breaking changes (renamed classes, new indexers, changes in XML configuration files, etc.) you must carefully amend possibly module overrides you have in place.