Will a spherical f10 mirror be pretty close to a parabolic f10 mirror in performance?
A 4" f/10 mirror could be made spherical and give some very good views.
A parabolic 4" f/10 mirror will be just a bit better.
On the 4¼" f/13 I made a couple years ago, I made it as close to a parabola as I could. At f/13 it too, could have been left spherical, and would have been a very good mirror. I got the correction very close to the theoretical values. It was an excellent mirror.