You know what, I still do 4 A levels and I wouldn't change it for the world. I do maths and 3 sciences, I enjoy all of my subjects and am enthusiastic in working towards all of them.
Doing 4 rarely gives you an advantage (it depends on the course and the university) however if you feel like you could cope with the workload and are already getting good grades, I say go for it.
Just ENSURE that you like all of your subjects. There's nothing worse than becoming demotivated half way through the year.
Ps. for the people saying that 4 subjects is too much of a workload, I disagree. It keeps me busy and I think if I only did 3 than I'd be bored half the time.