Read this post for instructions on upgrading a 3.4.x install to 4.0.
Just because you can upgrade doesn't mean that you should upgrade. Read the "Should I upgrade to 4.0?" post before deciding whether to upgrade.
Before upgrading, please perform a complete backup of your site.
Then, follow the instructions below to upgrade to 4.0.
1. If you have files in /wp-content/plugins/wpallimport/upload, move them to /wp-content/uploads/wpallimport/files/. “wpallimport” and “files” won’t exist yet – so create them manually, and chmod them to 0777. If you don't have files in this folder, you can ignore this step.
2. Remove WP All Import 3.x and all add-ons from your site by de-activating and deleting them from the Plugins page of your WordPress admin.
3. Install 4.0 and any add-ons you need the normal way – download the .zip files from the customer portal, and install them just as you would install any other WordPress plugin.
4. If anything in the plugin user interface doesn't look right, clear your browser cache and/or refresh the page.
That's it - you're now running 4.0!
Please double check your import settings and templates (Edit Options and Import Settings links on the Manage Imports page) and verify they are correct before re-running an import or allowing a cron import to run.