I went from 300 to 600 then did a 400 a month later.
If you are comfortable with a 200 then you will be fine with a 400 in my opinion.
Yes, the saddle may start to get uncomfortable.
Yes, your hands may get sore.
Yes, you legs will ache for a few days afterwards.
Yes, the night stage can be a bit dull and miserable if it rains.
Yes, you may get a bit sleepy, cold, bored & hungry at 3am.
However, if you can complete a 200 then you proved that you can keep turning the pedals for 10-12 hours. It's unlikely you will suddenly lose this ability to keep turning the pedals.
Unless it's a very hilly 400 then just ease back 1km/hr, keep eating and drinking and you'll be fine. Save any sprinting for the final 10m rather than the first 10km.
A 400 does not feel like finishing a regular 200km Audax and then doing another one (I always feel like "That's enough" after any Audax). On a 400 or 600 you'll find that 200km passes by surprisingly quickly.
I'd say that the sign that you can do a 300 or 400 is sitting in the finish of a 200 thinking "That wasn't too bad at all".
MattH makes a good point. I call it "NDA" (No Dicking About). Time can evaporate at controls and it's easy to settle in to a nice conversation with people who are faster than you and can spare the time. I still haven't fully committed to the NDA principle but, as MattH implies, any extra time in a cafe equates to less time available on the road. Importantly it can equate to more time riding in the dark and rain. NDA during the day allows you to dick about longer during the night when you really need it.
If I can think of 7 Phases of NDA then I will write a self-improvement seminar/book/DVD and make some money.
(I now fully expect YACFers to contribute the 7 Phases of NDA on an Audax)