When Thor goes to the pool to re-dream his vision given to him by Wanda Maximoff in Avengers: Age of Ultron (2015), he sees the Infinity Stones: the Mind Stone, the Power Stone coming out of the ball we saw it in in Guardians of the Galaxy (2014), the Space Stone coming out of the Tesseract, and the Reality Stone being coalesced from the Aether. However, he does not see the Soul or Time Stones, even though the four we do see float onto a cloud suspiciously shaped like Thanos' gauntlet. Why doesn't he see them? Is it because the writers did not know where the other stones were yet? Thor later speaks about the "6 Infinity Stones" when talking to Vision for the first time, but he only saw 4.
It’s a mysterious vision, not Cosmic Wikipedia. Only four of the Stones are in play at that point.
The Time Stone has presumably been relatively safely-guarded by the Ancient One ever since she was the Middle-Aged and Looking Good For It One; and the Soul Stone is safe and sound on Vormir, getting merely an occasional and only-slightly-covetous polish from Ol’ Schmitty.
In contrast, the Space Stone recently got stolen by Thor’s brother, who used it and the Mind Stone in an attempt to conquer Thor’s beloved Midgard. The Reality Stone nearly killed Thor’s girlfriend, and the Power Stone nearly destroyed Xandar. They’re the currently-relevant ones.
Thor mentions six Infinity Stones when talking to Vision because he’s King of Asgard, and knows what Infinity Stones are. (As does Lady Sif in the mid-credits scene in Thor: The Dark World.)