A solar system has 4 planets very similar to Earth, all in the habitable zone. The only differences between them are slight variations in size and therefore gravity. The tectonic plates and continents are also different for each.
There are two rocks that have existed since before early humans evolved. They can teleport between the planets and take along any life form touching them (Meaning if a person was touching it it would take the person and their belongings, including there micro bacteria, diseases and viruses). They also take any material in solid or liquid state that is on top of the stone touching it or within 10 meters above it. The rocks are immovable (relative to the planet they are on) and can not corrode or be destroyed. The two rocks always arrive on opposite sides of the planet, near the equator. One always appears on land and the other in an ocean. This means some species have evolved to migrate between the planets. The rocks are oval shaped, 10 meters long and 5 meters wide and 3 meters high, They are dark grey in color.
Every year they teleport to the next planet in the cycle. The order is A,B,C,D, then repeat. This is how animals and humans came to inhabit all the worlds. The term year is being used as every exact full rotation around the sun (so not the same as our calendar year). All the planets rotate at the same speed. The planets are located like this:
My question is what would society look like in medieval times, as humans now have access to over 4 times the amount of land to mine, farm, hunt, etc...