I have a V4 box and the recommended USB FTDI converter. I soldered pinheaders to UART and BOOT, so connection should be good and used short jumper cables
During flashing/reading i was not able to successfully read the box, i always got read errors inbetween. After trying a lot i remove the power cord and network cable of my notebook, so notebook and box ran on battery - and then it worked fine.
This is a reproducable effect with my setup - also i assume this will not be generic issue i just wanted to drop a note here, so in case someone has ongoing issues it might we worth to try as a last resort let everything run on battery.