I would always back up my best ds game save file as I don't want to lose it due to physical damage, corruption, or go without time based events from dry internal battery. I was also wondering if the same thing could happen to my save file on an r4 card. Would I be able to retrieve it? Also, if internal battery runs dry on the original ds game and I back it up, will the it be the same if I put the save file on the r4, or am I able to use time based events again.
The save file simply exists on the SD card inside your R4 flash card. So you can simply copy this to your computer to back the save up on the R4. DS games don't have an internal battery, so you'd never loose a save game like that. DS games use the clock built in to the DS for time based events. If you put a DS game in a different DS / modify the time, then time based events in certain games might not happen for 24 hours (Pokemon for example) however after that they will function as normal.