I put 5 because that's by far the norm in engineering at my school, and it's not at all unusual to add a non-science course on top of that. 4 science/math and one non-science course is sort of common, but anything less than that and people will start being curious about why you're doing so little (are you working full time on the side, or what?). You end up spending a lot of years in school if you don't take 5 most semesters, so that's why most people take on such a heavy workload. I've done 5 science courses every semester except for one, and I've been able to be successful academically. I've also taken my fair share of courses for physics/math majors, so I can tell you that the engineering courses are certainly comparable in difficulty to the physics/math major courses, the engineers just tend to take more science courses.
That said, taking 5 science courses is a lot of work, and I wouldn't do it if I knew I was going to have to deal with that kind of stress for more than a few years. So in the "is my life sustainable long-term when I'm taking 5 science courses" sense, I can't really "handle" 5 science courses. For a few years though? Definitely.