Yeah, recent research showed that BF alongside introduction of solids had benefits in preventing food allergy which might be why 4-6 months for BF. Also with FF, there is iron fortification so the nutrition from food isn't as important.
I took part in a twitter discussion recently on weaning and even though 'official' advice in the UK is 6 months with a reluctant "ok, if you must but not before 17 weeks", most dieticians in the discussion were advocating starting weaning between 4 and 6 months.
I think one thing which is worth mentioning is many kids do have growth spurts at 4 months and it is very hard keeping up with that as a BF mother. Some kids get past it, by 19 weeks, my son's growth spurt was still going so I started solids. It didn't make a difference immediately but by giving solids as well it did give me the confidence to do PUPD. Had I not done that, I would have been giving formula so it's an interesting argument.
Interesting article. Thanks for posting it.