Sorry Shelley, I don’t have any specific recommendations for beyond Volume III. There is huge number of possible pathways. My hope is that after completing the three volumes of BFSU kids will have developed an interest in what area(s) they wish to pursue further and there is a plethora of texts in each. My advice, then, is to go to Amazon, search texts in areas of interest, read reviews, and make choices accordingly.
Thank you for your question. Please let us know what you find, decide, and your future progress. I am sure that many in the group would like to hear. Best, and stay safe.