Will this ViewSonic LCD Monitor work w/G4 MDD?

It will definitely work through the DVI connector.

It should at least give you 1280 x 1024.
Some might give 1920 x 1024, depending on the graphics card present. Apple menu -> About This Mac -> More Info under AGP should tell you the graphics card model.

Here are the recommended Hertz that should work for higher resolutions:

http://ati.amd.com/products/radeon9000me/radeon9000prome/specs.html

If something you require has a Hertz level corresponding to the resolution given that is not listed, then you'll have to look at lower resolutions for that Hertz level.

I have the slightly larger VX2235wm which is 22 inches, and the same video card as you on my MDD. I am satisfied with its performance.

The monitor has separate VGA and DVI input ports. The max widescreen res on my MDD is 1680x1050. I usually run at 1440x900 because my eyes aren't what they once were.

I've gotten higher resolution on the same monitor by connecting it to my MacBook Pro via DVI. Can't remember what the numbers are exactly but I know it's larger than the 1680x1050 on my MDD. The notebook has a newer graphics chipset and 128MB VRAM, so that's not a surprise.

As an aside, I played with both the VGA and the DVI sides of the monitor. I found I have more options for smaller resolutions if I use the DVI-to-VGA adaptor that came with the MDD and connect to the VGA side of the Viewsonic.. I keep the DVI cable attached but tangling over the back of the desk so I don't have to rummage so much when I need to connect the MacBook Pro. It works fine on the DVI cable. As long as I turn off the MDD before connecting the cable, leaving both lines running to the monitor has caused no problems.