Anyone can get purple drops, if:
(a) they're fighting something from which they can earn xp/inf, which is
(b) not grey to them (since greys give trivial inf, but cannot drop recipes/enhancements/salvage) and
(c) capable of dropping a level 50 common recipe.
A level 47 character fighting even-cons therefore has a chance to get purple drops, because a level 47 mob can drop a level 50 common IO recipe.
A level 50 running a level 47 mission, however, needs it set to the third or fourth difficulty in order to make sure the minions con green, rather than grey, since as noted above grey-cons cannot drop recipes.
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Can a lv 47 get purple drops set to the 4th diff?