I was able to get some used dock for cheap. I have 2 3ft wide sections and 2 4ft wide sections. They have the alluminim siding and wood slats. Wood needs to be replaced. I am asking if a 3ft or 4ft is a better width to avoid sagging? We like the width of 4 ft but I’m worried that will start sagging more/quicker than the 3ft. Attached are pics. If both will last about the same without sagging then we would lean towards the 4ft. My personal opinion is the 3 ft woukd work much better.

It will not let me attach pics. The dock has the alluminum rails on the side and wood slats across. There is no aluminum center bar just a wood stringer