When to start baby rice/porridge etc

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My little girl will be 4months on the 12th and I'm not sure on when I should start her on baby rice etc as if seen a few that are 4months+. Obvs with all that's going on it's going to be difficult to speak to health visitor etc. Like how much would she need a day? When to give her it and what about formula? Shes currently on 7oz every 3hours.