Someone hit the jackpot in Season 5 and found a Mythic Unique with 4 Greater Affixes!
The Mythic Unique that dropped was 
Ring of Starless Skies! and it came with the following Greater Affixes:
- +26.2% Attack Speed
- +18.8% Critical Strike Chance
- +18.8% Lucky Hit Chance
- +3 to Core Skills
On the Season 5 PTR, Rob salvaged about 100 Uber Uniques to craft 20 
The Grandfathers and only 1 out of the all came with 1 Greater Affix.
Another player salvaged 40 Mythic Uniques to craft 10 
Harlequin Crests, but none of them had a Greater Affix.
This goes to show how incredibly rare it is to see a Mythic Unique with a Greater Affix on it, let alone 4!