Do it dude!
I have that large 4 piece and a medium 2 piece
My buddy has the medium 4 piece, I say it would probably hold like 1.5g comfortably, and like 2g tight in there.
To be honest though the medium isn't that medium. If I saw the medium I would think it was the small.
If you can spend the extra $ I'd get the large. You'll love it if you plan to use it a lot.
Like I said before I grinded an eighth in like 30 seconds and there was still a good amount of room in there. Could probably hold 5g-6g.
Also it has a lot more surface area on the screen, so it collects keif like a motherfucker. A LOT more than my friends medium.
Go to
www.vapeworld.com check out their space cases and their dimensions, then get a tape measure or a ruler and draw out the dimensions so you can see how big they are.
The medium feels REALLY light, and has some thinner metal because it is smaller, my large is nice and solid.
(solid website, ships out quick, order accepted/shipped emails sent, and a tracking number is included.)