The check box on the first page of the installer is there so that customers acknowledge the risk before installing Nik Collection 4 by DxO.
If you have a pre-DxO version of Nik Collection installed (from Nik Software or Google), Nik Collection 4 by DxO is going to fully replace that version, and it will no longer be available on the computer. The warning is to explain that DxO cannot provide pre-DxO Nik Collection the ability to retrieve your old plugins as they unfortunately no longer exist. We highly encourage you to create a copy of the installer used to install your legacy version of Nik Collection prior to installing Nik Collection 4 by DxO, so that if you do not like Nik Collection 4 by DxO, you will be able to try and install the legacy version from your old installer. However, please note that should this reinstall not work, we unfortunately are unable to offer further support for any pre-DxO version of Nik Collection.
If you have a previous version of Nik Collection by DxO installed, Nik Collection 4 by DxO is going to fully replace that version, and it will no longer be available on the computer. Fortunately, should you decide to use your old version, instead of upgrade, you will be able to reinstall this old version from within your customer account as long as it is compatible with your current OS version and your host software.
Please also see this article here: https://support.dxo.com/hc/en-us/articles/115015301507