Right well, I am really sad and love researching, so I have spent the last few hours looking up stuff to do with weaning! I think my personal stance is that i wouldn't automatically start weaning just cos they are 4 months, but neither would i steadfastlet wait till 6 months either. Not sure if that makes me bias or not?!
I have found the actually study done by the WHO and also lots of other studies done with regards to allergies/asthma/obesity/IBS etc.
And it my humble opinion the two conclusions have been mixed and twisted and put into babycare books.
The WHO concludes that there are no significant problems with exclusively breastfeeding for 6 months, along the lines of growth and length, and that there are benefits to do with intestional infections and losing weight in mothers. (which is a problem for developing countries, but helpful for overweight mums in developed countries!) They found no benefits of introducing food between the age of 4 and 6 months, apart from improved iron in some, and they felt that could be given via vitamin drops instead.
I have looked at quite a few other studies with regards to allergies and everyone of them says that these allergies are caused by introducing food before 4 months. I didn't find one single study that says they have found introducing food between 4 and 6 months causes allergies.
It would seem people have read the WHO encourages exclusive breast feeding for 6 months and that weaning early (they have took this to mean before WHO recomends,ie, 6 months instead of before 4 months) causes allergies and come up with the conclusion that weaning before 6 months is bad.
Also, the WHO findings only relate to exclusively breast fed babies. What of ff babies? I can't see that there would be any benefit in ff exclusively to 6 months for them? But i will look into it. I will post some of the links but want to post this before i lose it!