Are CO2 cartridges feasable for use on fat bike tires? Are CO2 or other portable cartridge style devices feasible for use on fat bikes with tires 26x4.5" or greater? I know there are large 20g or 40g cylinders available?  If so, what sizes of cartridges would be appropriate?

Though, I'm sure it might be good to have a pump as a backup as well in case you run out of cartridges.
The short version: Yes, they are feasible.

The long version: You can do a minor adjustment of my answer in this question by changing the molar masses involved. 

The molar mass of carbon dioxide is 44.01 g / mol. The ideal gas law says PV=nRT, where P is pressure, V is volume, R is the ideal gas constant and T is temperature (in an absolute scale, e.g. Kelvin). Rearranging terms and converting from mols to grams, we see (44.01 grams/mol)*PV/(RT) is the grams of carbon dioxide you need to achieve pressure P for a tire of volume V at temperature T. 

Lets be crude about estimating the volume of a tire using a torus. The volume of a torus is V=(pi*r^2)(2*pi*R) where R is the major radius and r is the minor radius. Google will calculate it for you (and has a picture of what major and minor radius is). We'll use an estimate of 15.25 inches for the major radius and 2.25 inches for the minor radius giving 1524 cubic inches, or V=25 liters. Assume a temperature of T=293 K (room temperature) and asking for a pressure of 1 bar (14.5 PSI) gives 44.01*1.02 g which is about 45 grams. We'll also assume the volume of the tire is independent of the pressure (which is reasonable). 

You can use this wolfram alpha link to play around with the numbers. Note that this calculation is linear in the pressure for fixed volume, so asking for half the pressure requires half the CO2. 

Alternatively, rearranging the terms, if your cartridge has M grams of CO2, you can rearrange  (44.01g / mol) *PV/(RT) = M to see P = MRT/(44.01 g / mol * V). Thus, the pressure you can achieve changes linearly with the mass of CO2 you use provided temperature and volume remain fix. 

So, if you want to go to up to around 14.5 PSI, carry a 45 g cartridge. A 20g cartridge will give you 14.5*(20/45) PSI = 6.4 PSI, and a 25 g cartridge will give you 14.5 * (25/45) = 8 PSI. 16g gives you 5.2 PSI, 14g gives you 4.5 PSI and two 16 g cartridges (i.e. 32 g) would give you 10 PSI. 

Note that these numbers are a bit rough -- you might lose some pressure connecting the CO2 inflator, and we used an estimate of the tire volume. With more accurate numbers you can improve the estimates (and they should be quite good with a good estimate of the temperature, volume of tire). Overall though, I believe this calculation gives a bit overestimated amounts of CO2 required -- the volume is likely a bit smaller than the volume we used as an estimate. 

But they should be good enough for determining the right size of CO2 cartridge to use. 

Note that increasing temperature means you need less CO2 (while holding pressure and volume fixed). Increasing volume (i.e. using a bigger tire than what you calculated for) means more CO2 (while holding pressure and temperature fixed). Increasing pressure means you need more CO2 (while holding volume and temperature fixed).