You are doing fine! DD2 was like this until 4m and then she changed but that's the thing - they are always changing their sleep to suit their developmental needs. As long as she is feeding at least 5 times a day and doing lots of wet nappies she sounds fine to be sleeping that long.

Her weight gain seems great - mine didn't hit the 5kg mark until 4 and 5 months old! If she is happy, alert and wet then you are doing the right thing.