The 4 oz. tanks are older and most of them were sold through Brass Eagle.
Generally, the 4 oz. tanks are steel (versus Al-Mo alloy for the 3.5) and they are significantly heavier.
The shape is different and the 4 oz. tanks are slightly larger in diameter. I'm not sure if they exceed the 2" rule and since I sold my old 4 oz. I can't just caliper it but I know they're very close.
The 3.5s are under 2" in diameter and thus never need re-hydro.
Don't worry about getting a 4 oz. tank, if that's what you're wondering. They're heavier, older and are prone to corrosion. The 3.5 is a better choice if you're looking to buy, plus it's easily available.