Using the Steam App, it looks like you can only play 4 player... is that the case? it says in the rules you can do 5 player if one person plays the imperials, but when you load the app it only allows you to do 4 heroes is this because the app takes place of the Imperial forces?

Yes, the app is four player co-op.

It’s considered the third game mode alongside the competitive 1 v 2-4 campaign mode and skirmish mode that come in the box.

Hopefully that doesn’t turn you off too much, the original one v many campaign mode is still pretty awesome.

(Also, go B’s!)

The co-op app is co-op, so technically you can play with any number of players if you play it as a full co-op without strict assignment of one hero per player. (Even in the 1 vs many regular campaign all players are still involved even if their hero gets withdrawn.)

One of the players can also be the "app handler", who interfaces with the app and performs the imperial activations.