Yes, to the best of my knowledge, Calgon is EDTA. I'm not sure of the purity, but I think you can use it. You can always get it from Photographer's Formulary if you want the real thing.(posted 8292 days ago)You should not have any trouble using tubes to process your 4x5. Just increase the concentration of the developer. Nearly all pyro developers are quite dilute, so it's easy to double or even quadruple the developer (the actual developing agent)concentration and still work within your 100ml tube capacity.
I would recommend you begin by just doubling "A" as I said before (instead of, say, 10:10:1000, use 20:10:1000; if that's not enough, try 20:20:1000, or 30:15:1000, etc.). Test a sheet and see how it looks. If you really need to, add EDTA. But if you don't need it, don't complicate things.