*You should now have read **BobG's post above, which you need to keep in mind. I am going on without you now as briefly/abstractly as I can, because I may have time constraints, so whenever I can I will reply, if you direct to me. There are more than enough very capable forum members here to bounce your ideas off of.
You may now also, have looked to the systems I mentioned to review, as to their features price etc. You have asked several questions. Let us start with the rears/surround. If not made for floorstanding but bookshelf, I like your idea as to placement near ceiling on wall mountings, exact location, I feel you have good idea, but if in corners, leave room for speaker's ability to be moved adjusted. If floorstanding rears then set on floor, with flexibility of position. Same thing with suggested speaker stands. You will have ability to tweak in these cases better over in-wall, etc.
To the front, looking at your room size, I see no reason that a floorstanding speaker cannot be placed on either side of ent. cabinet, doors fully opened along with sub, as you do not want fronts too close to center anyway and they can be towed in or out slightly for placement. If floorstanding then you should consider having center near same level (as was mentioned) by placing center inside cabinet. Looks big enough to me, maybe wrong. If you go the small speaker route as first suggested as an option, you have the ability you have said of mounting speakers for rears, so fronts can apply in the same way on wall mountings, keeping in mind the ability to adjust without hitting wall. In which case then you could as a resort(not ideal) place speaker center on top, skewed toward sofa/focal point ears/ear level when sitting,etc. Another, is center in cabinet and speakers on sides at same level on stands. Best is floorstanding, front/rears, center, fronts at ear level to begin with, but sometimes compromise, with eyes/ears to future.
Just remember you have options and I like the 5's better than the 4's, and the speakers *BobG provided links to is better,bigger still. Think about it, discuss with forum, discuss with wife, ask questions. HT is basically the same as music as per sitting ear level settings of physical speakers. Hope any helps! Remember you have to live with your decisions, when all is weighed.