Having the 4th or even 5th will make this game more dynamic. Even more play styles. The challenge is how to balance this against the original 3 party setup. The detail below might not be ideal so it's my pleasure for expert players to chime in: 1. This new bigger party needs to have similar total HP as 3 person team 2. Same total numbers of card of 3X37 for MP (or the same specific number in SP) This is achieved by using a slider of character level that increasing one level for a member reduces the other. Especially for #2, player will notice the number of 'slots' remain the same however they adjust this slider. 3. For each additional character you have in party, you got an additional move card per turn. This overall increase of mobility is balanced by more frail characters and there is increased contributor to the loss of victory point whenever your party member dies. 4. The main problem is that you draw additional 2 cards for each member. This can be resolved by drawing 1.5 cards per turn instead of the standard 2 cards. Yet, there maybe some more elegant way. Card balance will be changed somewhat as mass/burst spells will be more popular. To name a few: team heal, ember, frost cone, volcano, fire storm etc.
Small decks are much more powerful than large ones, that's why you don't lose slots when you're de-leveled for adventures/quests. Currently, the game always rounds card draws up, so you'd still get 2 per member. Dropping it to a flat 1 would mean you draw five cards per round instead of 6.
True, small decks are actually more powerful. Then the extra guys will use the same normal deck. Then there is no need to modify the 'level' of party members, which is good, streamlining this style of play. To achieve 1.5 card per round, the game can changed to draw 1 the first round then draw 2 the 2nd round. Trying to find a way that does not too artificial. Another option for a party for 4, 1 member does not draw a card in the 1st round so the total remains 6 cards per round. On the 2nd round, the next member does not draw any card.
I was going to say that 4-5 is probably less fun than 3 because it is slower and less 'on the edge'. But as an option with drawbacks, as I think you are suggesting, I agree it is quite an interesting concept.
You could keep most of the systems in place but just make specific adventures which require a 4-5 characters in a party. They could be these huge adventures with loads of enemies and a large map. I could see that being pretty fun. I reckon it would be way to cluttered having 4-5 players in the adventures currently being used. I guess multiplayer could work with 4-5 players, with some added rules. The only problem with multiplayer is that they would have to make more packs which include 5 characters. And there would have to be a separate lobby as 3 characters vs 5 isn't fair.
3 vs. 5 MP fights would be impossible to balance properly. However, this could be a custom game option, or there could be a special arena for this at some point. However, I'd rather see two new classes added before we have the option of 5 PC parties.