Thanks sincarafan. I went ahead and ordered one as well the other day. I figure since it has a rotating clip, it takes the place of a horizontal case.
If it has that much room in it, I might go ahead and get a Seidio Active case. With the kickstand, it will add some bulk (which I don't want) but it doesn't come any other way... yet.
I've tried the minisuit bumper... very minimalistic... too minimalistic for me. And also have a minisuit Kinnect case coming. At only $5 each shipped, I figured it was worth trying.
But I'm glad I have my GS4 in the Speck case. I dropped my phone onto a tile floor last night as I was dragging it out of my back pocket. The end caught my pocket just as it came out, pulled it from my fingers and it clattered to the floor. Completely unfazed... and undamaged. Even the case shows no evidence of being dropped. So I was glad I had something more than the minisuit bumper on.
Edit: my holster arrived today as well. I have my GS4 in the Speck CandyShell case. While it fits somewhat loosely inside the holster, it is not banging around and shows no evidence of wanting to come out on its own if carried in the horizontal position. Overall, I'm well pleased with this el cheapo holster.
Now to continue the search for the right case to hold my GS4... and which will fit into this holster!