Well, it does say "no fewer than 4". The vehicle might be able to launch more than 4 but it has to be within the capabilities of a Delta 4 Large or Atlas V. That will be the limiting factor. The Atlas 551 is supposed to be able to put 20,050 KG (44,110 pounds) into a 28 degree low Earth orbit. I'll WAG that to perhaps 38,000 to 40,000 pounds to the ISS. The Delta IV Large numbers are probably comparable.

Months ago, I posted about a briefing I attended on NASA's and the Air Force's vision for a future space plane. NASA and the Air Force were studying to see if they could make a vehicle to meet their needs. NASA wanted a vehicle capable of carrying 40,000 pounds and a crew of about 7 into orbit. They didn't care if it took months to turn the vehicle around for the next launch. In other words, they wanted Shuttle II. The Air Force wasn't even convinced it needed a spaceplane, but it thought a smaller vehicle (possibly unmanned) capable of being prepared for launch in a few days would be better. There wasn't much room for commonality between the designs. It sounds like NASA's new OSP proposal is more in line with the Air Force's plan than the agency's own wish list.

We don't need to send humans to launch satellites or even space station modules. A human carrying vehicle should be designed for that purpose, say a crew of 5-7 and perhaps a few thousand pounds of supplies. A vehicle that size shouldn't be any heavier at launch than a larger business jet like a Gulfstream IV or V. With that reduced orbiter size, the total weight at launch could be kept to a million pounds or so. Weight snowballs very quickly in spacecraft, and aerospace vehicles cost by the pound.