yes you could add to the tail end of it, although not sure if would go through the trouble of adding 4 ft. and then having move the axles etc,etc,....perhaps if you were only adding say 2 ft. maybe you wouldn't have to move the axles. does your trailer have a complete flat bed? I had a 12 ft. trailer with flat deck and was a bit short, as well as difficult to load with short ramps, so I add a dove tail of 2ft. to the rear and give a bit more room as well as easier to load, I have a CUT tractor and at the time thought I could get buy with just adding to the trailer, when hauling the bush hog the tail wheel would still hang off between the ramps and i had to rest the bucket on top of the front rail, I made do for a while, But many time I found myself needed another implement and had no room to transport it,

Is when I decided to trade the trailer for a larger one 16 ft, although it was also a flat deck and I modified it into a dove tail, didn't ad anything to it just mad a few cuts and drop the tail and reinforced it,
so yes adding to the rear can be done, but to limit your expense and effort of moving axles, you might have to limit the length of add on

the picture shows where I add to the rear extending the top rail & floor and add a diagonal bracing, after adding the rear cross iron 2 x 3 plus shifting the weight of the ramps further back had the trailer almost equal weight over the axles, I could easily lift the tongue when empty, Note: which is not really a good thing
