I think it's a Good Idea. I can see where you'd be all keen to do it G.

I think (no professional basis here) that the critical issue is rack
prevention. For your 4' lift I would try to leave 4' of leg in the
tube. Just seems right. Also, the lower point of attachment to the
container is vital for rack prevention. Instead of a foot up from the
bottom, I think attached to the bottom would be way better. If it
starts to rack, it could run away and be REALLY exciting.

I think that the 5 degree inclination could be a problem. I figure that
after being extended 4', the legs want to be 8" further apart. How are
you going to handle that?

As far as backing a trailer under it: how wide is the trailer compared
to the container? How much margin?

Good luck,
Bob