Here are some answers…
1.
2.A A vase, or a fountain, or a wine glass
2.B A hose
2.C A heating duct (rectangular cross section), A car tire
3. Show white on an RGB monitor, and a white piece of paper lit by a broad-spectrum light bulb: to a tri-chromat, both will look white. To the tetra-chromat, the monitor won’t look as white.
(note: this assumes that the 4th cone is far away from the centers of human cones – a better approach would need to know what the 4 cone colors are, and find combinations of those 4 colors that appear the same when one is removed)