At least a couple of issues at play if Bolt does take a serious crack at the 400m:
First: As Nanny has indicated elsewhere, the most successful 400m coach in history John Smith says (if I’m quoting accurately) the immediate future of 400m preparation at the highest level is not speed-based (consider that 11.5sec to 12sec for the final 100m is considered world class 400m split), one reason he has introduced repeat 1km runs during phase one at least of his GPP. Nanny could elucidate on the philosophy behind John’s current programming ideas.
IN any case, if John is correct, it may take a considerable shift in training focus for Bolt to accommodate the higher aerobic threshhold element indicated for 400m success at the WR level.
Second: Bolt will have tremendous speed reserve if he continues training for the 100/200 as in the recent past and may be able to rewrite the rules of 400m preparation if he does go anywhere near the 400m WR.
These are interesting comments from John Smith.