I know that the FDS is Japanese but for D-4 Any% and D-4 Warpless, why do people use the Japanese version?
For Lost Levels Warpless, there are no meaningful differences between Japanese and North American. S&Q is 73 ( I think) frames faster on J, but last I checked, those frames were removed from final time for any% D-4 runs on US carts/rom. People who run other games on SMAS like the J-cart, specifically the J 1.0 cart, for faster reasons in those other games.
They use the Japanese version because you can grab the flagpole backwards and it saves some frames
Following investigations regarding the accuracy and integrity of FCEUX, the SMB1/2J moderation team has decided that FCEUX will not be permitted for future submissions, effective September 17th, 2023. The accuracy of FCEUX has come into question following 108Pi's discovery that the current SMB1 Warp