Four countries as a World Cup host will be too much because that would be very unfair to the other teams who will have to qualify and have a hard time to go through the qualifiers of which 32 teams will be only accepted. I think 3 countries will be the maximum line, although there's no written rule for this, it will be a common sense not to have more than 3 hosts to remain fair.
It's just that Canada-America-Mexico agreed to make a bid together to increase their chances and indeed, they succeeded and they are the first ones who will host the world cup with 3 hosting countries.
I agree with your argument. At the most, only one team should qualify to the main tournament as hosts. Else European Union will host the tournament sometime in the future and will demand automatic qualification for all of its 27 member nations. There needs to be a clear-cut rule one the number of nations that can qualify directly. If 4 nations are hosting the tournament, then let them play a quadrangular series among themselves. The winner from that tournament can qualify directly, while the other three teams should be asked to participate in the qualifier tournaments.