No, you cannot upgrade Photos separately to Photos 4.0. Photos is part of MacOS X. It is your a wrapper for calls to system services and Photos 4.0 needs macOS 14 Mojave.
You can create a downgraded version of your Photos Library to use on on your High Sierra Mac with the help of iCloud Photos.
- Let your Mojave Mac upload the Photos Library to iCloud.
- Then enable iCloud Photos Library on your High Sierra Mac for a new, empty Photos Library. The library will download from iCloud to your High Sierra Mac and create a local copy that is compatible with Photos 3.0 on High Sierra.
With your Photos Library in iCloud you can use the library from Mac with different versions of MacOS X.