Are there any limits for Number of players in online coop?

[PLUM-XBOX-BUS]

Can I play coop with 4 other players? 

[Retrogamer125]

You can play with 3 other players online including you making 4

[JFwibbles]

It's important to know that the difficulty increases depending on how many players there are when playing online. The enemies' HP, speed and attack power are multiplied depending on the amount of players.

1 player = 3x (yes, the same as 4 players)
2 players = 1x
3 players = 2x
4 players = 3x

If one person disconnects after the mission begins, the difficulty multiplier remains at the level that it was at when the extra person was present, until you either beat the misson or retreat.

The takeaway here is that leeching is especially bad when there are 4 players, as the rest of the team will have to put in considerably more effort to take down the stronger enemies. Also, 2 players can clear missions much faster than 3 or (especially) 4 players, all because of the enemy HP multiplier.