Aluminum won't save all that much weight over steel with such a small trailer. Maybe all-aluminum will weigh 150 lbs less than all-steel? The cost, fab time, and susceptibility to cracks doesn't justify using aluminum in my opinion.
Also, if it is too light, the trailer will hop all over when lightly loaded. I recently bought and reconstructed much of a 4' x 5' steel trailer. The guy built it with standard 2000 lb trailer springs. Lightly loaded, the trailer seemed like it would hop 2 foot off the ground when driving down forest service roads at a decent speed. Now, it has 3" lift front springs from a CJ5, shocks, 37" tires with 10 psi pressure, and 3/4" plywood bolted over the sheet metal floor (50 lbs more weight). It only seems to hop 1 foot off the ground.
I would not build an all-aluminum trailer from scratch, but I might build 1 with a steel frame and aluminum panels bolted to it. It would be powder coated with the inside bed-lined.