Acoustic Treatment on rear wall

First of all, I don't have a state of the art home theater system. It is one I built myself and I think it does fairly well for a budget system. Currently, I have most of the corner issues addressed with either book shelves or plants. I don't have a problem with the bass. The first reflection points are either addressed with an open door on one side and a coat rack on the other.

However, I would like to address reflection off the wall behind the couch where I sit. I am a few feet away from the rear wall and I would like to put some kind of diffusion on that wall. The question is how many panels? I thought I would put up a 24 X 48 x 4 panel. Would that be enough for at least some kind of diffusion from behind?
Correction, I would like to add absorption on the rear wall and not diffusion.

Thanks in advance.
hi, my room was very boomy and had standing wave problems. i put michael greene room tune pillows in the 4 upper corners and 4 24x 24 inch cathedral panels[absorption] on the rear wall. problem solved for my room. due to having magnepans i put diffusion on the wall behind the speakers. with conventional cabinet speakers i put absortion behind the speakers. there are lots of sites for information. aurelex has one as do many others. you could also build some. good luck. my room is 14 x 22 feet
If there is room behind the couch, more books or records on shelves might do the trick.
I don't think you need 4" panels behind you. They are mostly for treating bass problems. I would use 2" and a couple at least rather than one.
I have a wall behind me like you. I have used a couple of GIK 244 bass trap panels (2'x4'x4") on the wall directly behind my head to eliminate the bass null caused by the wall. Works very well. If you are near Atlanta you can borrow mine to try.
Look on the ASC (Acoustic Sciences) website. I have used 8 of the 9"x 36" tube trap half rounds behind the listening position with excellent results. ASC also offers the Matrixpanel for this purpose. The Matrixpanels come in two sizes, 24"x24" and 24"x48". If you decide to make panels yourself make sure you use only natural materials. Synthetic materials will cause a frequency imbalance.
a single Echo Buster, about 48x4 should do the trick.
Rockadanny, I was considering a single GIK 244 on the wall behind. I was thinking that not only would it kill the reflection back there but might do a pinch of trapping some bass even though I don't really have a bass issue. I was wondering just how many I would need and it looks like either 1 or 2 would do the trick.

The space behind me is not big enough to put a book shelf or anything like that.

Thanks everybody...
Matchstikman - Yes, one or two GIK 244s may work well for you as do for me. Good luck!
I have a large number of GIK panels and have experimented a lot with the back wall. I currently use one 244 panel way up high in the corner (vaulted ceiling) and three echo buster panels mid wall. Since you say you don't have a bass problem, I think the 244 panels will over-damp. The 242 panels would be the safer choice.