Of course you can shorten the beams and add more legs. This stuff is pricey enough and doubling the legs and shortening the beam would double the price.
No, a 4 foot 1x4 is not as strong because of the MDF core on the I-beam. A 2x4 would be stronger.
If you were climbing on your layout there is a good chance that at some point all of your weight would be on the middle of one beam. The way the plywood attaches wouldnt offer a whole lot of load distribution to other legs. Dont forget the additional weight of scenery and such!
I didn't do this to be exact. I did this because if it HAD worked, then I'd spend the $2k on the stuff. I'd really hate to spend this money and find out it won't support my overweight self. This was a true worst-case test and man if it passed I woulda been so happy! I actually bought 2 beams and the legs were not damaged, so I could do some more testing but I'm pretty sure 280lbs is not going to be supported.
Like it was said already, this is off topic. This is not about my layout. I hope I helped answer the question of can you walk on top of this stuff. The answer is "maybe."
Now...back on topic...hey John, you running trains yet?!
