Can there be more than four active threats in play at once?
There are exactly four scouter-shaped slots on the side of the board with which to place threat cards. There can also be up to four players/characters, and the number of threats in play is implied to be equal to the number of players. However, I cannot find anywhere where the rules explicitly state that the number of active threats are limited to these four spaces; it would simply be very unusual to design the board to have exactly four of these very deliberately-shaped notches if there was such a common scenario in which they would overflow.
The rules for adding new threats are:
At the beginning of each round, reveal a number of Threat Tiles from the top of the stack equal to the number of players (e.g., in a 3-player game, reveal 3 Threat Tiles), then place those Threat Tiles face up next to the Cell Board.
Does this mean that, if you are playing a 4-player game and do not clear all of the threats in a turn, the uncleared ones linger around and four additional threats appear, exceeding the space on the board? Or do you fill up to four so as not to exceed the very perfectly-designed card spaces?