add support for GPT-4
fixed in other pr
GPT-4 uses the same ChatCompletion API as `gpt-3.5` before it. However, this API is only "switched to" by llms/openai.py when the model starts with `"gpt-3.5-turbo"`. This makes it impossible to set the model to GPT-4 and have it work properly.

My PR fixes this by changing the starts-with checker to accept all 3.5- and 4- versions.

source on model APIs: https://platform.openai.com/docs/models/gpt-4