Head to your local big-box lumber yard, buy two pressure treated five step pre-cut wood stringers made for decks, have them cut a 16' 2x12 into five 36" long pieces, buy a box of 3" deck screws and assemble the whole thing together with a piece of 2x4" under the first step
that will sit on top of your top folding step with a couple of screws through the underside into the 2x4" to hold in place. If the step is a little too high just trim off some wood off the bottom but iv'e built several of these and they all fit three-step fiver stair heights, also I added an 18' length of rope lights under the threads near the front edge zig-zagging from the bottom tread to the top using wire staples to fasten them which lights them up really well at night and with a folding hand rail they work great also un-screw them to stack in the bed of your truck for transport!
Red Green;
Remember, If it moves and it shouldn't, duck-tape,
if doesn't move and it should,WD40!!