We have the Sprinter based LTV 210B 3S that sleeps three comfortably with a 3rd seat behind the passenger seat that converts into a bed. This works fine for three (unless we are talking tall people) and I have used it twice for Alaska trips with teenage grandchildren. And it fits in most parking spaces except parking garages due to height.
But your requirement for four adult campers will be hard to meet in a Class B and the suggestion earlier in this tread to consider pulling a small trailer for the times you need four beds is the most likely to work and provide you with a vehicle that isn't dominated with beds and therefore marginally livable. I have considered this when I have contemplated trips with more than three and would go that route if one of these trips became a plan.
So far our Sprinter Class B has met all our needs and we love it. But your needs will be hard to achieve in a Class B.