Credits are charged anytime you ask a question and receive an answer. An answer from ChatGPT consumes 1 credit and generation of an image consumes 2 credits. You can purchase more credits by tapping on the credit balance button in the top right corner.
Currently, there is an option to enable GPT-4 in Settings only for people who have purchased Lifetime Unlock and are using their own OpenAI API key. I hope to add support for GPT-4 using credits soon.
This means you are getting a 429 error back from OpenAI. This likely means that you are using a free API key that has reached its limit. If you log into your OpenAI account and make it a paid account then you should be able to create a paid API key that won't have any limits and will charge you as you go.