People often think that not sleeping through is somehow being connected to weaning. Add to that the fact that in the past people often weaned at 12 weeks or even earlier, so older relatives can suggest this without realising that NHS advice has changed.
I wouldn't follow suggestions from friends over NHS advice. It's there for a reason, as our best understanding of the available research to date.
Particularly, rusks are not a good food for babies (no idea why they are so bloomin popular still). They are often full of sugar, and contain wheat which should be avoided before 6 months (due to the increased risk of coeliac disease, and possible allergy).
I do think though that it's worth remembering the NHS advice is to start weaning "around 6 months", when your baby is showing the signs of being ready (stick to the list on the NHS website as there is a lot of misinformation out there about supposed "signs"). So it could be 5 months 3 weeks, or 6 months 2 weeks, depending on the individual baby. It's highly unlikely that a baby at 4 months is actually ready for weaning.
Fwiw, I started doing BLW with my ds at about 5 months 2 weeks. He was 2 weeks overdue (no idea if this is actually relevant, but he always seemed to be a bit ahead with physical milestones) and extremely strong, could sit unaided and bring food to his mouth.