Hi @Gary
Welcome to Microsoft Q&A Forum, Thank you for posting your query here!
I see that you are asking how to use GPT-4 models in OpenAI Studio at Japan East
. You have also mentioned that you can only use GPT-3.5, and you are looking for guidance on how to use GPT-4 models.
A separate application for Azure OpenAI GPT-4 is no longer needed.
GPT-4 and GPT-4-32k are now available to all Azure OpenAI Service customers. Availability varies by region.
As mentioned in our below docs for Japan East,
Due to high demand, availability is limited in the region.

<sup>1</sup> Due to high demand, availability is limited in the region
<sup>2</sup> Version 0314
of gpt-4 and gpt-4-32k will be retired no earlier than July 5, 2024. See model updates for model upgrade behavior.
<sup>3</sup> Fine-tuning is not supported for GPT-4 models.
So, If you don't see GPT-4 in your region, please check back later.
https://learn.microsoft.com/en-us/azure/ai-services/openai/concepts/models#gpt-4-models
I hope the solution is useful to you and then accept the answer.
Regards,
Janarthanan S