But the benefit from using the API is that you can change the model on the fly, so you chat with 3.5 until you notice that it's not responding properly and, with all the history you have (probably stored in your database), you can send a bigger request with a probably better response once with GPT-4 as the selected model.
I really wish the interface on chat.openai.org would allow me to switch between models in the same conversation in order to 1) not use up your quota of GPT-4 interactions per 3 hours as quickly and 2) not strain the backend unnecessarily when you know that starting a conversation with GPT-3.5 is efficient enough until you notice that you better switch models.
OpenAI already has this implemented: When you use up your quota of GPT-4 chats, it offers you to drop down into GPT-3.5 in that same conversation.
