Vendors have long been a decent source of legendaries. It’s always been a good idea to glance at their inventory when riding through town, as they could have a great legendary roll now and then. However, in Season 4, they’ve upped their game!
As spotted by richardpwechsler, vendors can now have Greater Affixes on legendaries!
Source: PSA: Check the vendors! They now stock legendaries with multiple GAs!
As you can see, they’re pretty easy to spot and they can have multiples, so a perfectly rolled 4 Greater Affix legendary could be waiting for you! So whenever you’re not in a rush, click on a few of them and maybe you’ll get lucky.