yes i used this plugin, you will only be able to use the openai api for a month or so with a free account. if your account is older you can only use gpt3.5 via the openai website.
you get billed by usage, if you have an account set up to work with gpt4 (which you have to pay for) you automatically have api access in gpt3.5 and all other paid stuff you can see here: Pricing
gpt4 by api is not yet publicly available.