Hi there, So worrying having a child who is low on the charts. I'd say, some do catch up and some don't. Some are probably perfectly healthy tracking along the bottom of the charts, some aren't.
My dd was always on the 9th centile or just under during her first year, EBF, 75th centile for height. Health visitors gave me loads of grief, but she was bright, alert, met all her milestones, and after the first year, climbed steadily to 50th centile for weight where she's pretty much stayed. Told myself I'd never worry about weight in a baby again.
Then I had DS1! Born between 0.4th centile and 2nd, slipped below the chart and stayed below til had massive intervention (strong reflux drugs and NG tube then gastrostomy). He also didn't meet his milestones and although bright and alert, didn't sit or crawl til way over I year old.
So I would say, if she meets her milestones and stays on the 0.4th centile, then try not to worry. If she continues to fall behind on the chart or starts missing milestones, then I would push for a referral to a paediatrician initially.
It's also worth checking what her 'genetic potential' is - the red book tells you how to do this - it's a calculation of her final height range using a mix of yours and her DF's heights. So if you and your DP are small, she might well be within the range of normal for her. If you are both tall, then something may be a little awry.
Good luck. Very much hope it's just a blip and she climbs steadily back to where she should be.
Oh, and I would second waiting for weaning til 6 months. Milk has far more calories in it than initial weaning foods. This was the strong advice for DS1.