The '54 resto pack (should be the same for '55.1) shows that the 4400 axle is the same as the 36-38-4100. The dimensions of the I-beam, king pins, and spindle are all the same. The 4000 even uses the same wheel bearings as the 36-3800, though the hubs would obviously be different. The only other difference is that each series has a different weight capacity (2500, 3500, and 4000 lbs. for the 3600, 3800, 41-4400 series respectively). If the axles are all identical, I can't figure out how they have different capacities. The 6000 capacity is only 500 lbs. more, at 4500 lbs.
One note about using a '54 or '55.1 5000 or 6000 series axle--while the wheels may be the same size with the same bolt circle as the 4000, the wheel center holes are bigger. In '54, the front and rear hubs of the 5000 and 6000 series were beefed up, and the hub center diameter was increased 1/2".
