While the rules allows you to play with less than 3 heroes, there is almost no deck with one (before The Grey Wanderer) or two heroes. Why ? Because getting more ressources, more stats and more action advantage between your starting characters is more valuable than a lower threat. This contract does the opposite (trading threat for a 4th hero), and that's in my opinion what makes it so powerful : you start with 33% more stats/ressources than the average deck. What downsides will you have in exchange ?
With 4 heroes, keep in mind that your starting threat will likely be high. You will probably be forced to face ennemies like the well-know Anduin Troll on the first round, and every scenario focusing your threat level becomes riskier. You could already face this problem when building strong 3-hero lineup (like Gandalf/Elrond/Treebeard), but it becomes common with Bond of Frienship.
- Use low-threat heroes (Hobbits, Splorfindel, TÉowyn, even Sméagol)
- Have built-in threat reduction on heroes (Nori, Galadriel, Beregond, Frodo Baggins)
- Include threat-reducting cards. Because you have to include cards that don't belong to any sphere, Favor of the Valar is a must-have. Good old core set Gandalf too (note that you can't play hero Gandalf with this contract), especially because your increased ressource generation will make him easier to play.
- Embrace the Valor cards. Angbor the Fearless is a monster when your starting threat is already 40.
Resource & card management
You are forced to play a 4-sphere deck, with exactly 10 cards from each sphere. Having a solution to transfer resources between your heroes may be something you want. Denethor is your man in a gondor line-up. Errand-rider, A Good Harvest are probably some good cards too. And you can always rely on your 10 neutral cards to dump the leftover resources (sometimes even transfering them with a Envoy of Pelargir).
Keep in mind that with two copies of your more valuable cards instead of three, you will not always find them in your starting hand. So either don't play decks that rely to much on many-card combo, or increase the number of fetching cards.
You will probably have to make choices (i always end up with a sphere where trimming down to 10 cards is a pain^^), so you may want to look for alternatives in other spheres. No more place in for threat reduction or discard recursion ? You can give a try to (Nori, Secret Vigil).
Lastly, note that every card that gives an advantage to each hero gets better with this Contract (ex : Tighten Our Belts can net you 4 resources).
Playing with 4 heroes is really strong, so you could pick the best cards of each sphere and end up with some kind of Avengers team (i'm sure someone will build a One Deck with this contract someday. Edit: done).
But in my opinion Bond of Friendship will give you more fun when trying to build a thematic deck : Gondor or Hobbit are probably the most tailored for, but you can make a Bond of Friendship with almost every archetype of the game. The limitation in the number of copies will give you more variety by forcing you to include more differents cards, and you can end up picking some cards that where in your binders for a very long time :)