Hi. Make sure you ask for a pay rise to reflect added responsibility.
Given this and your other post tell the school you’re looking for a permanent job and ask about references. That way you won’t necessarily feel your Summer Prep is wasted. Or use Summer Prep time to find a job.
I have done a fair bit of long term supply the last few years. I do feel some schools view goodwill and flexibility as a one way street.
Look after your welfare. If it’s for 4 months it’s not for ever. Is it a fixed term contract or extended day to day?
Be prepared to leave with the minimum amount of notice you need to give.
I think some schools forget it can be quite difficult to get Supply Teachers in these days.
Ask other teachers in the school. Do they all lead a subject?
My inclination would be to decline - after all what will the school do when you leave at the end of four months? They should be looking at continuity - not hoping to string you along.
Sorry if this seems overly cynical. Also, I’m secondary so others may have more useful advice.
Take care