Yoko Shibui of Japan (2:19:41 PR- 7th fastest all time), a favorite to win the marathon at the upcoming World Championships, won the San Francisco Marathon in 2 hr 46:34. Is 4 weeks enough time for her to fully recover and be ready?
The Japanese are known for their long runs and high mileage, so a 2:46 should pose no problem for Shibui in 4 weeks time. In fact, there is some anecdotal evidence she may actually be stronger!
Ever hear of the Marathon Manics? These people clearly have rewritten the standard guideline for marathon recovery (one day for every mile raced before your next race- so 1 month minimum between marathons). Test any of these manics 3 or even 2 weeks following a marathon and no doubt you'll see minimal if any after effects.
I don't believe any of manics are challenging world class times, but that doesn't mean elite runners can't run marathons 4 weeks apart. Check out this bio on Mary Akor and note the dates on her string of marathons (Akor finished 19th in the 2008 Olympic Trials marathon). Most of her marathons run on 4 weeks or less recovery were faster. I've also experienced this the few times I've run them that close together.
Running 26.2 miles does do damage, no matter how slow or fast you run, but when your muscle cells repair they may actually be stronger or more tolerant of the beating a marathon will give them.
One final note: Shibui commented on the hilly San Francisco course, which can lead to even greater muscle damage relative to a flat course. It remains to be seen if this will translate to an even better race for Shibui in Berlin.