If a tooth that had a root canal 4 years ago was shaved down by a dentist, would the tooth have been broken during the shaving? would the tooth be more vulnerable to breakage given that it was shaved 4 years after a root canal?

While root canal treatment removes diseased pulp tissues from the tooth, the tooth does become more brittle and subject to fracture. Often the Dentist or Endodontist that accomplishes the root canal treatment will recommend that the tooth be crowned to add additional support. If the tooth was prepared (shaved) 4 years ago it was probably in preparation for a crown. Follow yo9ur Dentist's advice.

You've asked multiple questions regarding a tooth which apparently has a fracture, and you appear to think that your dentist is at fault for this fracture. Teeth that have received root canal therapy are brittle and prone to fracture. Not your dentist's fault. If the tooth is compromised, extract an place an implant.