A lot has changed from when the fourth edition of the Inform Designer’s Manual was released. Would a fifth edition be warranted?
The DM is very much in Graham’s voice. I wouldn’t recommend trying to modify it to a new edition that includes Inform 6.3 updates. It just wouldn’t fit.
(Graham could do it, but I wouldn’t recommend he do it either, because he should be working on I7. 
It would make sense to create updated reference material – the Appendices and Tables – and add them to inform-fiction.org/manual/.
Yes, absolutely the authoritative guide for Inform 6 should be updated if Inform 6 has changed. Having ancillary materials is often more confusing. People then are led to the canonical source (the manual) but then told to check the other material in case something has changed.
Zarf’s point is perhaps valid about the manual being in Graham’s voice – but so what? People aren’t reading the manual for a fictional experience. They are reading it for valid information about Inform 6. If updating a guide is based entirely on the “voice” of one author, that’s not a good thing!
The only reason I can think for none of this to be worth it is if so few people are using Inform 6 that the effort expended would outweigh the number of people who would actually benefit from it.
The writing style of the Designer’s Manual was what convinced me to try writing IF, back in 1999. It was clear to me that the fellow who had created Inform 6 was literate and clever. I probably wouldn’t have gotten into it otherwise. That’s my answer to “so what?”
In the years since, I’ve come to see that the DM sometimes fails to nail down the details in much the way that the Inform 7 manual sometimes fails to. There’s a tendency not to want to have the reader grind to a halt due to an excruciating level of detail. It would be nice if someone would tackle that challenge while also bringing the DM4 up to date (and being respectful of Graham’s voice). But I’m not volunteering for the job!
This is a well-meant and honestly rather flattering suggestion, but I think probably not.
The DM4 continues in all the essentials to represent the final state of Inform 6, whose primary purpose now is as a code-generator in the larger Inform tool-chain. We need to be clear, I think, that I6 is no longer mainly intended to be user-facing. Those users of I6 who do need to know what little is new since the DM4 would be better served by a change log than by scattered alterations to what’s a pretty large text.
It seems to me that it would be inappropriate to create a “fifth edition” when the changes made are so minor, and besides, in its very minor way, the DM4 belongs to the IF-cultural history of its times - which is to say, the turn of the century. So I think there are also artistic grounds for leaving well alone.
The way 6/12 behaves is more or less compatible with 6/11 and earlier, but it does add some significant new features. For instance, options to use first or third person narrative voices and a past-tense option. Could we perhaps revisit the question when I have all the kinks ironed out of 6/12?
Maybe would you consider a point release update of the DM4 that corrects the known misprints? I see something on page ii that suggests you’ve already done something like this: “Fourth expanded edition May 2001: Release 4/2 (July 2001)”