A 21-24 foot van has enough space to build a conversion with sleeping for four and highway legal seating for four, particularly if you are willing to use upper-level bunks and/or pop-ups.
Almost all the Westfalia conversions of VW compact vans sold in the U.S. would sleep four, as does the Traverse from Pleasureway, built on the standard length E-series.
What you sacrifice are some housekeeping facilities and storage space, i.e. you have a travel/camping van, not a complete little house on wheels. Closest to having it all was probably the Westfalia conversion on the Sprinter, sold here by Airstream. But even that was a compromise in that the bed made up in the seating/dinette space wasn't really suitable for larger people.
Among the larger B's, there are models that seat four and make up a couple of the seating positions into small beds. Winnebago Travato will seat and sleep four in two beds, but again, some of the sleeping space is for smaller people.
But the most B motorhomes sold are the ones designed for use by a couple, providing luxury living for two and maybe travel seating for additional people.
You have to figure out just what compromises you want to make, house facilities vs people space, and seek out the floorplans that most closely fit your needs. Having something custom built is certainly an option (I would start at Sportmobile, although there are other custom van conversion companies) but you will still be working with the space inside the van body.