That manual may be difficult to find for free. Try manual renters and dealers. I see the R4131D listed here: **broken link removed** **broken link removed**
Try asking Tektronix. They sell some Advantest instruments. They helped me find manuals for my R3265M.
FOUND IT! The Advantest site in Japan has it posted as a PDF file in the obsolete test equipment area. Be sure to select the ENGLISH sub-directory as the Japanese one is a little tough to read! There is a large selection of manuals for other equipment posted (also as PDF files)
Enjoy......