First, you might want to clarify as to what you mean by air raft. In looking at the way the air raft is depicted in Supplement 7: Traders and Gunboats, on page 24, the overhead image of the "Empress Marava", you have two different vehicles depicted. The overhead shot shows a vehicle of 3 meters by 4.5 meters in width and length occupying the air raft compartment. The first two 1.5 meter squares show nothing, the remaining four 1.5 meter squares show a single seat in each square, fulling occupying the volume of the vehicle.
On the other hand, the air raft shown in the side bar is quite different. The forward third of the vehicle shows what appears to be a closed compartment surmounted by a large wind screen. The approximate middle third of the vehicle shows two entry doors into the personnel compartment. The rear third of the vehicle is another apparently closed compartment, whether this is for limited cargo or part of the propulsion and life unit is unknown. The side bar image agrees with the air raft depicted from the side in The Traveller Book.
A couple of things are immediately apparent. The total floor area of the vehicle in standard 1.5 by 1.5 meter decks squares is six squares. The side shot shows a vehicle of perhaps 1.5 meters in height, giving a total volume of six 1.5 meter cubes, which would equate to a volume of 1.5 Traveller Displacement tons. That is far less than the listed 4 tons, presumably of volume in Traveller dTons. Now, you could allow for more than 1.5 meters in height, as the air raft in the overhead shot occupies a 3 meter by 4.5 meter compartment, and allowing for standard ship overhead, that would bring the volume occupied up to 3 Traveller dTons. That is again less than the 4 tons listed.
Then the air raft is listed at having a 4 ton cargo capacity. Now, this clearly is not in terms of Traveller dTon volume, but must be in terms of mass. Either way, given either the overhead or side images, where is this cargo carried? There is no room internally for it. Therefore, it must be the maximum slung cargo that can be carried. That in turn implies sling cables and cable anchors on the air raft to attach them too, with the cables being long enough so that the can be attached to the cargo pallet or container when the raft is grounded.
Oh, and by the way, no forward propulsion units are shown anywhere, unless that is what is housed in the rear compartment of the side image.
Now, trying to come up with a weight of the air raft, what we have to work with are the dimensions, 3 meters by 4.5 meters by roughly 1.5 meters. That gives us a floor area of 13.5 meters or 145 square feet. I apologize, but my brain, and my reference materials, are still working with English units. Looking at the 1953 manual for U.S. Army vehicles, I find a few that would work for comparison.
First, the good old Army jeep, at 11 feet long and 5 feet wide, one Jeep would cover 55 square feet, approximately. So three Jeeps placed sideways would occupy about the same volume as one air raft. Each Jeep would weight 2450 pounds, so three Jeeps would weight 7350 pounds, or 3.33 Metric Tons. Although that would include the power plants and drive trains for 3 Jeeps, which might be more weight than the power plant of the air raft. It does make for a first cut, though.
Then there is the Cadillac 3/4 ton Metropolitan Ambulance. At 7 feet wide and 20.5 feet long, it has a ground area of 143.5 feet, quite close to the air raft's 145 feet. It is just under 2 meters tall, so it is a bit taller than the 1.5 meters we are assuming for the air raft. We do have only one power plant and drive train when it comes to weight, and the ambulance is fully enclosed. The empty weight of the ambulance is 5720 pounds, or 2.6 Metric tons. This might be closer to the weight of the air raft, although it is fully enclosed. Perhaps you could view this as the weight of an enclosed air raft, with the un-enclosed air raft weighing closer to 2 metric tons. Now, the ambulance does have a 3/4 short tons cargo capacity, but that is nowhere near the 4 Traveller ton capacity of the air raft. As the 4 tons capacity of the air raft cannot possibly refer to Traveller dTons, one must assume that it refers to mass. I am not sure where you are going to put 4 mass tons of cargo in the air raft as depicted.
Given what we have to work with, it is a conundrum that is not easily solved.
