Hi Jazz B,
Mounting all speakers on the short walls 4-5 feet from the corners, (with the room being approximately 3 times as long as it is wide) will concentrate more sound at the ends of the room leaving a bit of a "hole" in the middle; the speakers would be approx 9' apart from each other on the room's short walls but 55' apart down the length of the room.
For more uniform sound distribution, mount 2 of the speakers near the corners BUT on opposite long walls and mount the other 2 staggered down the long walls, as follows: (consider the long walls as N and S...and the short walls as E and W)
1st speaker - mount on N wall at approx 7' from the NW corner
2nd speaker - mount on S wall at approx 21' from the SW corner
3rd speaker - mount on N wall at approx 34' from the NW corner
4th speaker - mount on S wall at approx 48' from the SW corner (i.e. 7' from the E wall)
(Simply: length of the room divided by speaker quantity and staggered across the room evenly from the end walls and down the room. With 4 speakers to utilize, the fact that the room is so narrow in relation to the length contributes to the speakers being much closer to each other.)
Yes, the omnipolar design should be mounted upside down. With that said, the Mirage FX model (I have Mirage OMD-5's and OMD-15's at home) appears to be a reasonable-quality speaker. But in a room the cubic volume (LxWxH) of yours and mounted at 16' AFF (above finished floor), there will be limited output volume and bass at listener level. Nonetheless, without knowing what model your ceiling speakers were and where they were located in the ceiling, I'd still guesstimate that the new sound will be lots better...and more evenly dispersed
Cheers.
Mounting all speakers on the short walls 4-5 feet from the corners, (with the room being approximately 3 times as long as it is wide) will concentrate more sound at the ends of the room leaving a bit of a "hole" in the middle; the speakers would be approx 9' apart from each other on the room's short walls but 55' apart down the length of the room.
For more uniform sound distribution, mount 2 of the speakers near the corners BUT on opposite long walls and mount the other 2 staggered down the long walls, as follows: (consider the long walls as N and S...and the short walls as E and W)
1st speaker - mount on N wall at approx 7' from the NW corner
2nd speaker - mount on S wall at approx 21' from the SW corner
3rd speaker - mount on N wall at approx 34' from the NW corner
4th speaker - mount on S wall at approx 48' from the SW corner (i.e. 7' from the E wall)
(Simply: length of the room divided by speaker quantity and staggered across the room evenly from the end walls and down the room. With 4 speakers to utilize, the fact that the room is so narrow in relation to the length contributes to the speakers being much closer to each other.)
Yes, the omnipolar design should be mounted upside down. With that said, the Mirage FX model (I have Mirage OMD-5's and OMD-15's at home) appears to be a reasonable-quality speaker. But in a room the cubic volume (LxWxH) of yours and mounted at 16' AFF (above finished floor), there will be limited output volume and bass at listener level. Nonetheless, without knowing what model your ceiling speakers were and where they were located in the ceiling, I'd still guesstimate that the new sound will be lots better...and more evenly dispersed

Cheers.