it's a 33% increase compared to doing 3 A-levels, which people already find taxing.
Personally the only time I think it's ok to do 4 A-levels is if they include maths and further maths - it's less work than doing four completely distinct subjects, as you would be doing. If you apply to university, you will be given offers requiring only 3 subjects.
Personally I think you'd regret it, plus you'd have fewer free periods at school to do work in.