I did my PPL in four weeks in the UK in the summer of '82. But then the PPL only required 35 hours (but it took me 38). With the JAA PPL requirement now being 45 hours, I doubt that anyone can fly and learn that much in four weeks. I did the PPL when I was 17 and was flying two or three trips a day, and studying for the ground exams when not flying.
Could you get 10 hours or so under your belt before the summer hols, say flying at the weekend, or in the evenings. If you do this, you would be sure to go solo in the first week of the four weeks - this would seem to guarantee compeleting the course in four weeks. But no one can guarantee the weather for you in the UK.
If you have passed all the ground exams by the beginning of your four weeks, you will eliminate any possiblity of being held up by these - you have to have passed the Air Law Exam before you go solo, and many schools like you to pass the Nav exam (and possibly Flight Planning and Meterology as well) before going on solo navigations exercises.
Having completed the RT practical test before the course will also put you ahead of the game.