Not to be argumentative either.... and not intending to confuse the matter any more but we've got a 4" a 6" and a 6' long bed sander.... the kind f*ctories use for shaping necks.
The 4" is great for nuts and saddles and small stuff and I keep 120 paper on it to cut down on the depth of the scratches in my nuts.... and saddles as I approach final thickness. The 6" has 80 grit and is great for larger stuff, making bridges, brace stock, having the piece ripped from your hand as you burn your fingers and then sucked into the General dust collector...etc. Don't ya love going though a dust bag to find that customer's bridge.....
The long bed sander although a coveted thing we have used so very few times that we don't even want it and would like to sell it. These things are pretty large and take up a LOT of space and when only used once in a while the space is more important to us than the convenience of being able to play guitar f*ctory worker with necks. Anyone want a long bed sander, you haul and the price is cheap...
Anyway the point of my post is that I think that there are great uses for either a 4" or 6" and if you keep different paper on them both it's great to have something that can sneak up on a final thickness or hog off material like there is no tomorrow.
Some of the long standing members will recall that I started building guitars in a spare bathroom and a Residence Inn in Sunnyvale California that I stayed at during the week for a year....
Business travel never kept me from building stinkin guitars.... The maids tended to hate my guts though.... 
If I had to go back into the bathroom shop or just a simple basement shop building guitars I would keep the 4" and nix the 6". It's amazing how much value you can learn to get out of a tool if it's all you have and thinking back I think that I had more fun building back then too when the instruments were the goal not a woodworking shop....
But, as always, to each their own.
It lives on the shelf, though.
![[headinwall]](./images/smilies/headbangwalluf8.gif "Mad")