Advice on Doing Four/Crash Advanced Highers?

Hey, folks. I'm starting my 6th year of high school on Monday, beginning my Advanced Higher courses, and am having a few reservations about my choices in terms of the workload it will place on me. My plan is to spend the month before summer tackling this workload, and if it proves too much I'll speak with my school about changing my courses before the beginning of the new year in August. However, I thought I may as well also ask for the opinions of other students who have been through Advanced Highers themselves and therefore know the sorts of workloads involved.

I'm currently planning to - and my school is allowing me to - study English, Physics, and Pure Mathematics at Advanced Higher level. These three subjects I am studying through the normal progression route (SG -> Higher -> A.Higher). On top of this, I am also taking a fourth Advanced Higher, which is History. I haven't taken the History Higher course, however - it's a "crash" Advanced Higher, although I did do Standard Grade History. I have done well enough throughout the year in all of my subjects for my teachers to allow this, although they took some convincing, despite them describing this sort of workload as "unprecedented". >_>. I'm not getting cold feet about it - I look forward to being challenged next year - but I want to try and start getting a better idea of precisely how heavy my workload will be beyond "a-hell-of-a-lot".
As a side-note, I've also been approached by an RMPS teacher I used to discuss philosophy with about beginning an Open University course in Philosophy aimed at high school students. I'd like to do this too, although I strongly suspect it will be beyond me to take this on as well.

If anyone with experience with multiple AHs or crash AHs could give me a vague idea, it'd be greatly appreciated! If it helps, my Highers are Psychology, English, Mathematics, Physics and Chemistry, all of which I've consistently received A-grades for in prelims and tests with little or no revision.