Would help to post all your specs, specially Case, GPU, cpu cooler, and what do you have on the 4th pcie slot?
Artic Accelero III (i used to run it before IV), its a 3 slot yes... but it has a mounting plate on the back that exceeds the back of the PCIe slot. I used to run it on TJ08-E with an HR-02 and it touched, even when bight a 3 slot cooler, with smaller CPU coolers like Mugen 4 or Megahelms you might be fine, although hard to confirm for sure as i dont own either, nor a micro atx with accelero III, but its my personal guess based on my past experiences. I also used to own a Galaxy GTX580 that had the Accelero III from factory, but it was design very well with backplate and didnt need the huge mounting plate the Accelero III Xtreme has, this was the main reason i opted for a similar solution on gtx750, but was a surprise the mounting plate on the back.
Now on options, there arent that many, look into GELID Icy Vision Dual Fan VGA Cooler
, Accelero Twin Turbo II
(not the new version with the heatsink on the back) and Raijintek Morpheus
(maybe with slim fans could fit...).
One important thing to note is that in most of the new gen cards, something in PWM design changed, the Accelero III had troubel controlling the fans on the GTX780, but it was fixed on the Accelero IV, but you might run into those issues depending on the GPU you will be using. For this alone, i would suggest you consider simply selling your current GPU and just get a quieter one with factory aftermarket cooler that its quiet, like MSI Twin Frozr IV or the new Asus that i forgot the new name of the their coolers.