Yes, you can use Custom Dimensions and Metrics in Google Analytics 4. In order to do this, you must first set up your custom dimensions and metrics in the event parameters. Once it’s setup, you can then use them in any of your reports, including in Summary Cards. Do keep in mind that the data related to these custom dimensions/metrics will only start to populate after you have created them, so you won’t be able to see historic data. You can create these in ‘All Events’ under the ‘Custom Definitions’ option in your Google Analytics 4 property.