Other sets of Horde rules use different knobs to turn to adjust difficulty to scale with player number. The player scaling is one of the challenges of doing Horde, since one wants to keep the format simple, yet fun enough for repeated play. Often the four-player game gets so easy that it's not fun after the second or third time.
Some reduce the life total of the players, so that large teams have less life. Note that the originally published rules have MORE life for more players, which is simply making an already easy game even easier. Reducing the life total doesn't change the stabilization rate of the Survivors and doesn't make the Horde any more deadly. It just lets some early-game luck on the Horde's part create an overrun once in a while.
Others adjust the total number of cards in the Horde Deck, adding more cards for more Survivors and making the deck smaller for fewer players. This again doesn't change the ease with which a team can stabilize. It will make the game a little longer, and add a couple more chances to run into a Plague Wind, but once a team stabilizes they're usually swinging through for 10-20 damage each turn to deplete the Horde of cards. This doesn't make the Horde any more deadly and doesn't combat the four-player stabilization problem.
Another rule is adjusting the token to non-token ratio in the deck. A Horde with more creature tokens is more deadly, since longer runs are more likely. This does make the Horde harder to beat, and combats stabilization a little bit since players may be forced into chump blocking before they wanted to. The disadvantage is the amount of additional setup and breakdown time, counting tokens to make sure you have the right number or choosing which weaker non-token spells to remove. One could perhaps combat this by writing on the front side of the sleeves what player number they should be taken out at. This factor makes token ratio scaling unfavorable by my evaluation.
We've just started playtesting a new rule for adjusting the difficulty of Horde Sealed for the number of players: The more players, the smaller each Survivor's card pool is. Our initial tests have proved promising. This may provide an elegant and easy way to scale by player number. There are more players to block, more mana on the board, more spells cast, but each has access to fewer high-quality cards, less synergy, and must have a more fragile mana base. This is a knob to turn that isn't available in Horde with constructed decks.
Our initial rules are 2 Survivors: 90 cards, 3 Survivors: 75 cards, 4 Survivors: 60 cards. In all the games before this, a four-Survivor team is VERY dominant and rarely loses. Perhaps this will take them down a notch. It may take them down two notches, we'll have to see.