Model Type Selection with GPT-4

[Oscerlot]

Just added in a change so that a model type can be selected, and you can pick between GPT-3.5-Turbo, and GPT-4.

A warning is displayed if you select GPT-4 reminding you that you need approved API access for GPT-4

Thanks!

[keijiro]

I'll consider adding it when I get an access to GPT-4 (I haven't gotten it yet).