Hypnosis requires a line of sight! ;)
although...
hey, which version of Heroes 4 did you play?
if I'm not mistaken, in the official versions, hypnosis and teleportation don't require a line of sight. But in Equilibris, the hidden stack with hypnosis, teleportation, and other spells that work in the "native" versions without a line of sight, can't be accessed.
We played the official version, and it definitely doesn't require a line of sight, which is why I still haven't been able to download and install Equilibris. What a pity! It's a good thing they introduced the need to see the opponent to cast hypnosis and teleportation. At least it's some kind of limitation to the "excesses" of Haven.:)) But in the official version, it's useless to try to hide, so I didn't bother.