So what is special about DO? I never really paid any attention to those lenses.
A DO lens element is able to bend light more sharply without introducing chromatic abberations. Canon's resin lens can do this, but it is apparently extremely expensive to build. They probably get a very small yield.
This equates to a very expensive lens, which is not going to sell, just to shave a little weight off.
it's not weight, it's size that it affects.
It affects length. That means less weight. The 400mm DO is shorter asnd much lighter, which makes it reasonable to handhold. The size and weight go together.
I just looked at Canon website,it says:
EF 400mm f/4 DO IS USM - 1,940g
EF 400mm f/5.6L USM - 1,250
EF 400mm f/2.8L IS II USM - 3850
It's significantly lighter than f/2.8 one, which is not a surprise when considering it's a f/4 lens, also its heavier than f/5.6 now I see no reason to buy DO lens unless its way too much cheaper.