How to purchase chat gpt 4 subscription for API?

We are using chat gpt 3 and we want to upgrade it to chat gpt4. How do we make purchase of it.
Please provide me guidance in this.

If you are already using GPT-3.5-Turbo via, then you just need to swap the model name out for GPT-4-Turbo or GPT-4o.

…and pay potentialy 60x more per API call.