In the dream world, if my BB has three ultimate skills, can I still understand the skills of the fourth grid?

No, the only way to expand the skill set is to prepare several multi-skilled and qualified BBs. When you are prompted by divination that you will have an accident when washing and summoning animals today, you will have a much greater chance of joining BBs in Liu Xian with more skills (more than four) or excellent qualifications. That's what my original BBs did.