failure to thrive? should i give my baby pablum? ?

my baby is about 4 weeks old and still not up to birth weight. she was strictly breast feeding but i put her on formula as well to help withh weight gain but it doesnt seem to be speeding anything up and im told if she doesnt gain it soon she will be failure to thrive. a friend of mine said to give her a little bit of pablum to help her gain the weight, she said she knows people who did it and it worked miracles for them but im just wondering if my daughter is old enough or should i wait and let her gain at her own pace and just hope things work out?