Why four loudspeakers ? Three might work.
In order to accomplish good, clean sound you need to carefully select ldspkrs based on architecture, seating layout, acoustics, distance, rigging, required SPL's, etc.
The biggest mistake folks make with do-it-yourself acoustic treatment is not realising that we need to tame reflected energy across a fairly wide frequency spectrum and therefore what is needed is not simply some curtains or music store foam wedges. In order to absorb lower frequencies ypu weill need thicker curtain/foam and to place it away from the reflecting surface.
The second mistake folks make is to simply assume that absorption is the key. It is not in many cases. Scattering the sound (diffusion) is often the better treatment.
I cannot design the ldspkr system or prescribe an effective acoustic treatment here and without compensation. No one can.
If you wish to solve these problems you need to hire a specialist in sound system design and acoustics.
