4 months & 6kg is a healthy weight. A couple of my bubs were around that size at the age. Like others said 'if she's happy & healthy it doesn't matter what she weighs'
With my bub#2 I barely weighed her at all, BF exclusively until 9 months simply because she didn't need solids til then. And I only weighed bub #3 more often because he was prem. I say wait & see what you DD is like closer to 6 months (if she's ready for solids).

You are doing a great job!