Just one simple thing you can do to punch up the shot. Is to always think about what is behind the heads that you will be filming. If they are standing in front of a black drape then in a medium shot all you will see is a head and black.
It usually a good idea for interviews and for speakers to have some sort of breakup pattern on the back wall to give some sort of texture behind the "talking head". This pattern should be extremely abstract and thrown out of focus.
You will want your light to be diffuse do create less shadows. If you can get some Lowell lights with soft boxes that may be all you need instead of the S4’s.
http://www.lowel.com/popular_kits.html
You need to focus light so that you have a front light, a rim light, and a fill light for every shot. This will keep the image from looking flat.
Your biggest issue with using S4’s will be getting a even enough focus. So if you cant get the lowel’s on stands then try to get some fresnels with some diffusion.
JH