Do we even know if these are actually unannounced, versus merely unverified? Not that i don't expect several unannounced Nintendo games to be there, but there doesn't necessarily have to be four of them.

Zelda Wii U, Animal Crossing Wii U, Pokémon Wii U (probably something in the vein of stadium/revolution/coliseum) and Advance Wars for 3DS.

Grand Theft auto city stories for 3DS - Is what I am predicting that big 3DS title will be.
http://au.ign.com/articles/2014/05/22/take-two-files-for-city-stories-trademark

It's also worth noting 3 upcoming exclusives for "Nintendo platforms" being announced coming from Natsume. No word of what platforms. Could be just 3DS.

Maybe one of Natsumes games? I hope it is a harvest Moon game for Wii U D:

True GTA on 3DS would be massive news and certainly worthy of that 90 minute roundtable they have planned.  But I think these promotional segments are for Nintendo developed titles.

How is it known that Nintendo have four unannounced titles to announce?

It certainly would be a title that would help the 3ds a lot in the west

WiiU Music + 3 ds games.

The broadcast schedule for e3 has 4 slots dedicated to unannounced Nintendo titles, beyond the regular e3 Digital Event.  We don't know exactly what that means, but it seems to imply that Nintendo will have at least 4 new titles to show that will deserve at least a half hour to go over.