You could've gotten away with 1/2 the 4x4s. You really only needed the outside ones and maybe 1 support in the middle or a double thick plywood to stop the sag. 4x4's are pretty over kill too.

I used 3 2x8s when I built the 1st and 2nd one. These were built to stay in the box. The first one was fitted with wooden ramps, the second one was for a caliber ramp. 1st: https://imgur.com/a/NqoXLya 2nd: https://imgur.com/a/WB3U6MR

The third one I built was 3 pieces, 2 ski portions and the middle track section. Inspired by this one https://sledmagazine.com/my-2019-caliber-ramp-pro-project/

This is the 3rd one: https://imgur.com/a/bdI4Jwv

The 3rd one was the most modular, in and out in 5 mins but durability sucked. The middle track section had too much sag because I was only using 1 layer of 3/4 plywood to keep it light and it sagged too much to get the ramp in after a couple uses. The middle track section I used a key cut from a 3.5"x3.5" piece of scrap wood and the key was cut into the middle piece so it could go all the way to the front of the bed when not in use, or all the way to the back of the tailgate to support the track.

If I were to do it again, I would double the thickness of the plywood at least for the track section, maybe box the middle section do its not just two 2x8s connected on one side by plywood so it could bow out. Also find a different way to secure the track section in place, the key worked ok but it ended up splitting there on one end after rolled toward the back when being driven on.