Hey! I got a 4 in my January mocks but got an 8 in my actual GCSE. My best advice would be little and often. I used 'maths genie' to master a topic. I did a new topic every few weeks and worked at a relaxed pace. The best thing about Maths Genie is you can start with lower grade questions and build up to the harder ones instead of throwing yourself into the deep end with the higher questions. The same goes for Dr Frost Maths online. And as everyone has already said, the best method to improve is just answering questions.
Try not to stress yourself out and just do small manageable chunks. Don't look to go from a 4 to a 9 in a week, but think about the big picture, maybe look to improve a grade a month or two. When you get closer to your actual exams start to do practise papers because those will help, but don't focus on those now.
If i had calmed down during year 11 and not stressed so much i probably would have had a better year. School hasn't even started and you are stressing like i was last year during summer! It will be ok promise, take a deep breath and do a small managable chunks of maths and you'll improve in no time.
You got this!