Where are you getting the 4'9 number? My ds will be 8 next month, and while he is still too short to ride w/ the regular seatbelt, I do think he'll be tall enough well before he reaches 4'9. I thought the rules were that they had to be able to sit on the seat w/ back against the back, knees bend at or past the edge of the seat, and seat belt crosses over their shoulder, not their neck? My ds is 50" tall, and I would say he is only about 2-3 inches away from meeting these guidelines. In total height, not just torso height. I think he'll be tall enough in a year or 2. By law, he won't be required to be in a booster after he turns 8 next month, but he'll be in one until he is able to fit the seatbelt correctly as I described above. I seriously doubt he'll be 4'9, though.
I just realized that the rear bench of our Sienna is shorter, in height from the floor, and depth, so that might be why he's so close to meeting the guidelines in our van. He definitely wouldn't be so close in the front seat. The seatbelts in the back seat also have built in shortener things, to make them lower.