Haveing 2 characters collected all the dvalin fragments and 1 character waiting on last fragment I can confidently and from experience say the order of drops are 001,002,003 and 004. There is no way you can track it but they usually drop during stikes or majority drops during public events like taken invasions or defending warsat where you get to kill a lot of enemies. The only way you can get it is to kill a lot of enemies in one session as I believe it increases ur drop rate. Also, equipping stuff like ether seeds when kiling fallen can help with the drop too! Hope this helps