So what, there's a sextant, the Piri Reis map, and the Phaistos Disc so far? Let's start with other historical artifacts that are out there.
As a Minnesotan, the first thing that comes to mind is the Verendrye Runestone. Allegedly found in North Dakota in the 1730s, it contained writings in a "Tatarian script" (Norse runes? Old Hungarian?) The alleged trail of the alleged artifact disappears in a cathedral in France which was bombed during the Second World War. That "Tatarian script" you could easily say was old Sperethiel.
http://en.wikipedia.org/wiki/Verendrye_RunestoneThat said, two of the artifacts have European origins so far, so it might be time for something else. My bet is that the fourth run was planning to be something from Qin Shihuangdi's tomb in Manchuria, considering how much they've foreshadowed about the tomb so far. We know the published run was going to take place in Asia, at least. And China is a very old civilization. Another Chinese option might be to go back to sites of the earliest known (or at least we think it existed) dynasty in China, the Xia, which was in SR's Henan. Given Aztechnology's involvement there and in the last two published runs, it's easy to think they might be involved in Henan too.
A lot of cultures I know less about, but you could easily have an artifact from South America, Africa or Australia that's not in the popular culture.
And then there's the Voynich manuscript, which is just asking for a treatment, although that's Europe again.