From the PHB page 66:
Starting at 2nd level, you can use your action to magically assume the shape of a beast that you have seen before.
And page 69:
At 10th level, you can expend two uses of Wild Shape at the same time to transform into an air elemental, an earth elemental, a fire elemental, or a water elemental.
It seems fairly clear that these are two distinct abilities, even if they share many of the same rules. The first allows you to take the form of "a beast that you have seen before", while the second lets you take the form of four specific creatures. There is no mention of needing to see those creatures before you turn into them.
It should also be noted that this only allows you to turn into one of those four, specific elementals, not elementals in general. So you can't use Wild Shape to turn into, for example, an Azer or a Galeb Duhr.
Finally, if your DM decides to rule that you need to see each of them to turn into them regardless, you can always cast Conjure Elemental four times to see each of them.