55g should be 48x13, but pretty much the same.
You should be just fine with 4 bags. You'll probably have some left over. I used 4 1/2 bags on my 90 (48x18) and it was plenty.
Also, I second what Ian said. Why is it that so many people use aquasoil ALONE if it doesn't contain high levels of nutrients (it does)? I use aquasoil with powersand and, yes, PS comes to the surface if you're not careful. I was a little sloppy with moving the substrate around and have to push down white pebbles every so often. No big deal though, it's a quick fix. The bottom line is, Aquasoil is head-and-shoulders above everything else I've tried (fluorite, gravel w/root tabs, sand w/peat). I'm getting stellar growth, less algae and I don't even dose every day like I used to! If you buy from ADA's website, you should be able to get 4 bags for $104 + shipping, I think. It's DEFINATELY worth it. I'll use it on all my tanks until something better comes out.