Curious what make up of 4 Rebels people have seen to be the most successful in Campaign? Which heroes have the best synergies together and thoughts on optimal combination of 4 heroes?

We're running out second campaign right now.

Team one was Mak, Gidion, Diala and Fenn. The heroes won nearly every mission but it went down to the wire.

Mak struggled to damage for a while but with Gidion supporting well (and giving the team cheaper Rebel Troopers). Fenn really shone against Stormtroopers. Diala was epic but got wounded nearly every round. She ran ahead and softened up the enemy.

Team Two (only three players though) is Jyn, Gaarkhan and Fenn first mission was very tough for them. Gaarkhan tanked ALOT of hits Jyn stayed in the midground and Fenn held back a space of two. Only one mission so far but they struggled.

Our rebels are running the same team as above, Mak, Gideon, Diala & Fenn. With the specific upgrades, they are impossibly strong: Mak kills an AT-ST in a single round, Fenn kills entire stormtrooper groups as they're summoned, Gideon lets the do additional things, and Diala lets everyone reroll stuff and do the whirlwind attack on groups. In this group the only problem is tanking. Fenn has too much HP but the rest of the team can die fairly quick.

This team also has an additional bonus; 3 of 4 heroes can move other figures around, which lets them move cruical NPCs, ending some missions prematurely. Also they can hide very well.

They have won 7 out of 11 missions so far, but in the finale, which we had to cut in half, they lost Mak and are struggling.

To answer your question; if you guys can get Luke at some point, I suggest going for Mak, Gideon, Fenn & Gaarkhan. Luke can mimic Diala's reroll support (which is cruical) without taking strain. Otherwise, go for the same team. (This team has way too much area effect, Gaarkhan has more single target oriented compared to Diala).

Gideon + Fenn + whoever.

There really isn't a Bad combination of heroes, merely a Bad combination for your preferred style of play.

I've had tremendous success with Mak, Gideon, Diala, and Gaarkhan in my last run. Near the end, the damage output of this group was tremendous, with the melee characters forcing the fight up front, and Mak destroying the tougher Imperials with his Pulse Cannon.

Though I admit that winning Temptation as the 1st side mission really helped. That way, nobody was fighting over equipment. One of the reasons I like putting Gideon in the group is because I never bother purchasing a weapon upgrade for him. His starter pistol with Stun is fine for the rare occasion when he's actually using it.

My friend likes to build his party around the no fought for equipment theory, including Jyn, Gideon, one melee and one rifleman. I prefer either two melee or two riflemen so that one can get the other's hand me down.

I run a mostly direct style though.

I was the Imperial player with four friends playing Mak, Jyn, Diala and Fenn.

They won early games, and I won the later games.

My "going easy or hard" on them seems to have been the difference, although dice are random result generators so it's hard to say for certain.

I feel like playing to the best of my ability would've changed those early game results (I pulled punches at first to test the water, and help them enjoy it more)

As for the heroes, Fenn would Havok any units standing too close.

Jyn would quick shot and run off.

Diala was a supporter, but non-optimal build (by player's own admission)

Mak was a high damage single target build, with a cowardly player who constantly tried to keep him out of harms way.

I suspect there are no really "optimal groups" for IA, but have no experience in how the other 2 (now 4 with Twin Shadows) play...