We started at 4mo on medical advice to help with DDs reflux. She was well and truly ready and has loved her solids from day one.
I think that the 4/6 mo debate will never truly be resolved... every person has their own reasons for starting solids when they do and its usually whenever their baby is ready, some earlier some later. IMO the major problem with current medical advice is that the studies are often conflicting. You get told one thing this month, something different the next.
Do whatever feels comfortable to you.