The weekender HE and any pot that is designed to house a 230gram\8oz fuel canister will be too narrow for a coleman 400. You need a set with a 135mm\13.5cm diameter. A snow peak 1400 set should work but might be too short. A friend of mine had a coleman stove in one of those, he had to force the stove in a bit but the lid did sit on top. I cannot recall if it was the 400 or not though. If the GSI dualist worked but was 3\8th too short you might try the stainless dualist. The lid on that is taller by roughly half an inch. Also, the pot for the solo stove titan may work nicely. All my suggestions are stainless steel aside from the snow peak which is titanium (but they make an identical set in aluminum), I do not like aluminum cookware. A 14cm Zebra pot should be a perfect fit. Those can be had cheaply. Post pictures with whatever you find works.
Greetings, Dominic, Our favorite cook set for a Coleman M400, is the old Sigg Tourist set that they made to accommodate that stove. The M400 nests in that Sigg kit, and the windscreen setup that came with the set does a very good job. Not going to be easy to find, unfortunately, but doing so will be well worth your efforts!! Good luck, and God Bless! Every Good Wish, Doc P.S. If it were me, I eschew all the smaller cook pots, as the M400 is a powerful stove, and needs to have a cook set with more capacity, IMHO. For what it's worth......
My Coleman 400a "packs" nicely into a cheap coffee (?) pot w/lid from a "Palco" cook kit. I'm usually using it on canoe trips with 2-4 other people, so the pot becomes an extra water-boiler to supplement the larger cook kit. Packed thusly it fits (barely) into the padded Coleman Peak One bag. (Would also be useful as a solo or hiker pot if water boiling was the primary use for cooking.) Tom