What a horrible shock, especially after being reassured by your HoD that you wouldn't be put in this position.
If it's making you unwell, would there be any benefit in your GP signing you off and occupational health getting involved to consider appropriate support? Or are other primary schools still struggling to find PPA cover and advertising jobs? Given your current school knows about your MH and it's an error at their end that has resulted in your needs not being accommodated, perhaps the headteacher would be willing to release you from your contract without the usual notice period?
If you are staying, at least for the start of term, are you able to share some of what the aims of your sessions/lessons are e.g. is it exam technique? Learning technical vocabulary? Pre-teaching of a concept? And how much flexibility do you have with the content? How are the students that come to you selected? I know at my DDs' school it tends to be students with dyslexia who aren't doing MFL along with those struggling with attendance and/or mental health who receive core subject support during MFL lessons.
As you love teaching Y4, you already know more bizarre SPAG terms than about 97% of the population. Likewise, your maths is likely to be better than a significant chunk of the wider population. Although you may feel uncomfortable teaching the KS3/4 content, could you KS2ify your sessions so you're closer to your comfort zone?
How would you feel about turning the situation on its head and givng the lessons more of a study skills focus or being a sort of 'learning surgery'? This might be something the school is willing to be flexible with to make amends for putting you in this position, it may be worth thinking about what you're willing/able to do to take some control over the situation and offer it up as a solution. For example, instead of trying to teach content, teach them how to construct mind maps, to take notes, create index cards, learn formulae songs from YouTube etc., basically anything that will help with revision? If going for the learning surgery option you might ask students to bring along something they are struggling with and as a group, consider ways to tackle it e.g. ask a friend, look on Bitesize, check YouTube, see if it's explained on Dr. Frost or similar, have a go at the relevant exercise on Seneca. You might want to set each of them to investigate one option and feedback to the group with their solution in ten minutes. Whilst the tricky thing being worked on may not always be immediately relevant to the whole group, the research skills, summarising and being able to extract sufficient information to pass on to a peer will be useful at some point. Get out the felt tip pens, sugar paper, post-its and 'round the world' maths games and give the students something completely different, more like primary school and recognising that they will learn much more from you if you feel comfortable and relaxed than if you're fearing for your mental health.
I really hope you find a way forward that works for you. It's horrible to feel trapped and let down.