Having never ridden a 400k, was wondering what one would advise to take?
Up to a 200km ride: Toolkit (multi-tool, chain tool, spoke key, leatherman mini with knife and pliers), puncture repair kit, 2 spare tubes, mini-pump, spare cash, spare chain links (for fixed), spare stuff bag (SPD cleat, various bolts), zipties, latex gloves (keep hands clean when doing any repairs), electrical tape, lightweight rain jacket (Montane Featherlite), phone, wallet, keys, lights mounted to bike, ibuprofen/plasters/antiseptic-wipes, spare brake blocks
Also, depending on the weather (rain/cold): swap jacket for a proper one (Gore Alp-X), knee warmers, arm warmers, leg warmers, thicker gloves
non-summer rides (where there may be night riding): add on Petzl head torch (emergency front light plus useful for repairs).
300km ride: add a folding tyre, maybe another tube, Portapow battery pack (to keep GPS and/or phone topped up). Spare lights (lightweight ones that would do in an emergency).
400km+: maybe another pair of mitts, Pro-Plus (may need to stay awake...)
Don't really add much on top of that for 400km or 600km.
For 1000km and further: spare jersey(s), shorts and socks.
Bonk rations (chocolate flapjack or the like) in little tri-bag behind the stem. For 300km+ rides I'd have something else, like a sandwich or pasty in the main bag. For some 600s (like the Bryan Chapman which has a 100km+ stretch at night with no shops/pubs/garages open) I'd be carrying a couple of bits of food just in case.
I have a Creek2Peak rackpack that's about 15L which gives me enough space for all of the above, plus spare space if I need to throw some food in there. Just resist the temptation to fill it before you've set off.
Two water bottles (750ml each). Topped up before leaving each control (after drinking whatever I need to at the control). Zym or Nuun tablets in the water to stave off dehydration and electrolyte loss.
Probably lots more I haven't remembered, need to start making piles of stuff for PBP.