Does the R4 use 'sav' files ?? - if so there's a good chance you don't need to convert to use on a DSTwo
I had 'sav' files from various cards like the M3,N5,Acekard2i & DSTwo & they were (mostly) interchangable (i.e. I used AK2i on a M3 - or a N5 onto the Ak2i etc)
....
I say 'mostly' because with the Acekard I did have one game file that would be seen on the M3 but not on the AK (mind you this was @ a time when Ak was using 'sav.nds' file & not 'sav' files)
Otherwise you could try converting using
http://www.shunyweb.info/convert.php
Although there's no Supercard DSTwo option - you could always try one of the other possible 'sav' types