I can't help specifically, but I assume the QT at the end of the number denotes Queensland Transport, and the '4' maybe refers to the home port. Assuming Brisbane is '1', it could mean the vessel was registered in Rockhampton. The official boat number should be etched into the main beam overhead. Generally near the compression post and often above access to the forward cabin.
I imagine there is a chap wearing a bowtie somewhere at the offices of the Queensland Maritime Museum, who would either know the answer to your question, or who would relish the task of searching for an answer.
Here is their contact email@example.com
PS. It could be worthwhile contacting a marine insurer such as Club. You can be sure that if you were insured and made a claim, somewhere, someone would dig up the original details.