I ran rank 4s for every element for several weeks until I ran out of elemental runes; when I'm low I run rank 3s instead. I've typically had a ~95% success rate in rank 4s.

Rank 3s seem to have an equal SSR drop rate to rank 4s. Rank 4s drop 2 SRs instead of 1, and more enhance fodder also. If you just want SSRs, rank 3s are fine. However, SRs will actually be standard equipment for quite a while. You'll get a lot of them, so you'll have a wide pick of buffs to choose from, and they're relatively cheap to level to max for the full buff effect. For the must-have affliction rate up buff, I'll even happily equip Rs. Meanwhile, I haven't leveled any of my SSRs; the best I have is a Water brooch with 2x HP and 1x Ability DMG. That's not worth leveling.