Loot filter for rare rings / amulets

[Titanium]

You should select "High" in settings > Loot > Stash settings.
Then you can edit "Rules\Hight\Legendary.txt"
There is already a line that's similar to what you proposed:
[TYPE] == "Ring" && [CRIT%] >= 3 && [CRITDMG%] >= 20 && [AS%] >= 6

You could change it to
[TYPE] == "Ring" && [CRIT%] >= 3 && [CRITDMG%] >= 30
to match your exact request of "4-6% crit chance along with 30-50% crit hit damage"