As the Galaxy Z4 does not run on Android software (It runs on Tizen OS), you can only use the Tizen store for downloading apps. It is not possible to download the Google Play Store for your Tizen phone.
Tizen is an open source (OS), created by Samsung in an attempt to reduce their reliance on Google Android. The Tizen Store is an App Store specifically designed for Tizen apps. To download apps for your Samsung phones, you will need to initially pass through the Tizen Store.