Keep in mind that on many tractor brands, R4 tires can not take wheel weights. Not all brands, but many only offer them for Ag, R1 tires.
With 4WD and a good heavy implement on the back for ballast, I don't believe filling is the best, or necessary option, for loading or other tasks.
RV anitfreeze, and windshield washer fluid (usually much less expense) are also good alternatives for filling if you decide to go that way. I wouldn't use CaCl or other toxic antifreeze at all.
With the work you're going to be doing, I would expect some punctures, so filling would be out of MY choices.
I don't have filled tires on my CK20, and haven't really needed them. If I need more weight for a job, I just do it with the BH on.
Most other times, my boxblade is sufficient ballast and I don't have to worry about all that weight when using the backhoe.
John