that is a very unrealistic goal for the summer - yes, it is possible to self-teach A Level Maths but not in 2 months. Also, you’d want to enjoy your Year 11 summer as much as you can because you won’t have a lot of time for fun during Year 12 summer (tbh this also kinda depends on what you want to do at uni).
Honestly, you do not need 4 A Levels. Unis mostly require 3 A Levels, they’d rather see 2 or 3 A*s than something like A*ABC.
You could do an EPQ in Year 12 along 3 A Levels.
You could also start with the 4 A Levels and see how you like it and if you’d be able to commit to all 4 subjects - if not, you can drop one of them at any time.
Or you could self teach A Level Maths in the 2 years you’ll be at college/sixth form and then sit the exam privately.