Sorry--I made an error; it appears that there is only one other study, the EAT study.
This study (LEAP), of course, didn't actually give us any information on whether 4mo vs 6mo is better; it just looked at babies who were started on peanuts in their first year or so versus those whose parents avoided PB till a later age. The EAT study was, I think, about starting various foods (sesame, wheat, egg, peanut) at 3mo versus 6mo, so that should shed more light on whether 4mo or 6mo tends to be better.
I have to admit, I am privately hoping that early introduction of allergens turns out to be better, for two reasons.
Firstly, it will be easier to control: if early introduction is better, then parents can start taking proactive steps to reduce allergy risk (whereas if LATE introduction turns out to be better, parents are stuck in this role of trying desperately to avoid any exposure at all for six months, which can be tough--lots of babies end up grabbing food or being given something by a "helpful" older sibling).
Secondly, the Kellymom types who relentless scold and patronize anybody who considers introducing solids before the clock strikes midnight on the 6mo birthday are just SO bloody annoying, and it would be amusing to watch them getting their comeuppance.
Still, nothing to do but wait and see. Let's hope that as the situation becomes clearer, we can start to take steps to reduce the incidence of these terrifying allergies and perhaps make more progress towards curing allergies in children who have already developed them.