Carpentry and Woodworking - Looking for advice on built-ins

Hello all, I am planning to install some built-in shelving on either side of my fireplace in the near future. Each will be 4' high by 4' wide and I will paint them white.

I was planning on using MDF, as opposed to plywood. Any thoughts on this?

Also, I plan on trimming the face with some poplar strips or something. Any thoughts on trimming across the top? I'd kind of like something like crown molding, but obviously that won't work still the shelves only go 4' high. I'm having a brain fart here trying to picture other non-ceiling height built-ins I have seen. Any thoughts?

MDF won't work for the shelves unless they have a lot of intermediate support, 4' is even a stretch for plywood, if they have to support any weight.

If the top hangs over, a small crown will work.

Yeah I plan on putting a brace at the halfway point of each shelf. In that case, it should be strong enough, right?

I'm also concerned about the look/texture, whether it will look quality.

I don't understand your comment about the crown.

Midway support should resolve my questions.

You suggested you wanted to use a crown trim for the top. If the top shelf overhangs by the amount of the crown, you could use crown. If not, forget what I said.

Gotcha. Not sure on that.

I guess I;m just having trouble picturing the trim on built-ins I have seen that are not ceiling-height.