ChatGPT plus is a consumer product, with subscription in a completely separate billing system. You already get GPT-4 access in the chatbot as one of the upgrades.
APIs use a completely different payment plan than ChatGPT. Different payment cards, different charge generation by use, credits and account balances, different web site domains, different purposes. For developers of consumer-facing applications.
If you are forced into a prepayment plan by never having the prior monthly billing system with card on file, the minimum credit purchase you can make is $5.