For a 4 month old, it might still be worthwhile getting one that has a sleep hood or neck support for when they fall asleep being carried.
You can go back to
www.thebabywearer.com and search for the mei tai that has the best review star rating. You can also join the Baby Carriers Down Under group (link in sling_dad's sig) and borrow some from the loaner database. I find that try-before-you-buy is the best way to make sure you get the one that best suits you and your baby.