Depends on the person, i know some people who stuck with it and got good grades, a lot of people drop their 4th one because its a lot of work and not everyone can handle it. A lot of the people i know who did 4, probably would have had higher grades had they done 3.
Personally i don't see the point in having 4 unless you are sure of doing REALLY well in them. Having 3 doesn't really disadvantage you.