Starting with 4 A levels then moving to 3 A Level plus 1AS

Planning to do eng lit, history, politics and econ next yr for a level. If it gets too tough I'm thinking about simply taking an AS exam one subject at the end of y12 and then only studying 3 A levels afterwards. Obviously my goal would be to complete all four but they're obviously tough but that's my backup plan (or just dropping a subject outright). Would that be better for applications coz I have an actual accredited grade?

Starting with 4 to see how you go sounds like a good idea. If you end up struggling with one, or realise you really dont enjoy one of them, then if gives you the option of dropping one and still carrying on with the 3.
Doing one to AS sounds like a good idea if your school will allow it. At least its something extra to put on your application, and it gives you time in year 13 to focus on other stuff.

not a clue if school allows it if they don't maybe I can enter as a private candidate

That’s not likely to be a cheap or reliable way to do it. After all, not all centres offer AS exams and even if they do for your subject, there’s no guarantee they’ll offer them for the same exam board as the one you studied. Just dropping the fourth subject and not sitting the AS might actually be the better option if it does come down to this.
I would ask your teachers or exams officer if they would allow anyone to sit AS exams. If they do, great. If not, then my recommendation would be to drop the fourth A level and not do an AS as unis won’t care enough for it and looking at the effort, cost and reward suggests taking it privately isn’t worth the bother.