4-5 hours a day is definitely enough if you use it effectively. You do not need to study 8 hours a day every day (some days you might study lots, but it is unattainable to expect you'll do it every day).
Past papers are the best way to revise, and as you go through them, take note of the topics that you struggle with. You can then make sure to target these topics by going over your flashcards for them more often and blurting.
Blurting is when you read your notes on a topic and then write all of the information that you can remember without looking at your notes. You then write the information you forgot in another colour pen and make sure to look at that more often.
For some subjects, like English or biology, making mindmaps for each topic or theme might be useful for you.
BBC Bitesize has notes for most subjects and mini quizzes on the topics in a subject (e.g., ecosystems in biology). Going over these notes and then taking the quizzes can be helpful.
I hope this helps, and if you have any more questions, feel free to ask me. 😊