The Fitbit Versa 4 currently supports only one voice assistant at a time, and that is Amazon Alexa. Despite being a Google product, the Versa 4 does not support Google Assistant, unlike its predecessors, the Versa 3 and Sense, which supported both Alexa and Google Assistant[1][5].
To use Alexa on the Versa 4, you need to set it up through the Fitbit app by tapping on your profile picture, selecting the device, and then choosing Amazon Alexa to sign in with your Amazon account[2][3]. Once set up, you can interact with Alexa by opening the Alexa app on the watch and giving voice commands without needing to say "Alexa" first[3][4].
If you want to switch to a different voice assistant, you would need to disconnect the active one first. However, since Google Assistant is not available on the Versa 4, you cannot switch to it at this time[7]. There is a possibility that Google Assistant might be added in the future, similar to how it was added to previous models, but there has been no official announcement from Fitbit regarding this[1].
In summary, you cannot use multiple voice assistants simultaneously on the Fitbit Versa 4, and the only available option is Amazon Alexa.
Citations:[1] https://screenrant.com/fitbit-sense-2-versa-4-google-assistant-support/
[2] https://www.youtube.com/watch?v=64oBC4DpgKg
[3] https://media.croma.com/image/upload/v1686496760/Croma%20Assets/Communication/Wearable%20Devices/User%20Manual/263858_User%20Manual.pdf
[4] https://documents.4rgos.it/v1/static/1153729_R_D001
[5] https://community.fitbit.com/t5/Versa-4/Google-Assistant-is-not-available-on-Versa-4/td-p/5301762
[6] https://community.fitbit.com/t5/iOS-App/Fitbit-app-doesn-t-allow-to-connect-multiple-devices/td-p/4015481
[7] https://support.google.com/fitbit/answer/14229424?hl=en
[8] https://store.google.com/us/product/fitbit_versa_4?hl=en-US
[9] https://voicebot.ai/2021/06/03/fitbit-makes-google-assistant-audible-on-smartwatches-6-months-after-alexa/
[10] https://www.reddit.com/r/fitbit/comments/1grnete/on_a_versa_4_can_you_open_google_wallet_or_alexa/