How did you cut the polygon cylinder cap/base, I tried using the "Interactive Split Tool" and the "Insert Edge Loop Tool" and neither of these would allow me to add edges to split the middle cylinder into a quater.Personally, I would start with the cylinders. Delete all but the top faces. Then cut the center one into quarters and just build 1/4 of the caster base by combining the quarter face and the whole face and then use a combination of bridge and append to polygon and extrude and target weld verts to fill in the gap and go from there...
From there it is just a couple extrudes to get to here ...
The "Cut Faces Tool" under the "Edit Mesh" menu seemed to cut the base into quaters.