4 months is NOT too young for a Holland Lop to breed. They mature quite quickly.
AND, if the buck squealed and fell off, he likely did his job.
Expect her to be pregnant, and take the proper steps. Get her a nestbox and some hay (grass, orchard, timothy - NOT alfalfa), and put it in her cage on day 28. It has been my experience that young, maiden does are usually a day or two late. She should normally deliver on day 31.
Expect her personality and attitude to change, at least until the babies are weaned. What you are seeing now is likely nesting behavior. She may become more aggressive and the nips turn into full bites. I would allow the daughter to handle her ONLY with strict parental supervision until the babies are weaned. (4-5 weeks old - take them away from her at 6 weeks old). Fingers / small hands stuck into a territorial Mama's cage will likely get bitten.
She will need a cage, by herself, in a quiet place. No dogs, No cats, No other animals coming up to sniff. Keep the buck away. He will re-breed her immediately following the birth. Her cage, food and water bowls will need to be large enough to accommodate her and up to 6 kits (more likely 2 or 3). Holland lops often have stuck kits because of their huge heads. Plan now in case a vet visit becomes necessary. Locate a rabbit knowledgeable vet, put the info in a readily available place, and save back some cash.
You might also start looking for homes for the kits. 6 weeks isn't that long, and it has been my experience that finding pet homes in the dead of winter is tough. Letting them go as "Christmas gifts" is a bad idea.
Also, the hardest part of rabbit raising. Steel yourself for if things go wrong. Maiden mamas are often not the best. There is a good possibility she will over clean the babies, resulting in missing legs, ears, or tails. Depending on how bad the deformity is, you may need to put down these babies. Also, if things go horribly wrong, you may end up losing the doe. This is always hard to explain to a child. Prepare now.
Hopefully, everything will happen smoothly and you will have beautiful babies within a few weeks, but, living in Oklahoma where things change in an instant, I always find it better to be prepared....just in case.
As far as the nipping, it's probably hormonal and should stop when the babies are born. If she starts to dig at you, simply remove her, or distract her with a treat or toy. Good luck, and remember, we always want to see pics, especially of new babies!