Four block starter iron farm

A very cheap iron farm for three villagers and one zombie. The zombie sits in a boat where the villagers can see him, golems spawn on the path blocked ground and get washed into lava over the hoppers. Four building blocks plus a handful of signs and slabs is the entire cost, so it is buildable on the first day. Java only, Bedrock has no zombie scare mechanic.

• Kill every iron golem in the area first, including ones in caves and ravines. A single live golem nearby stops new ones spawning, and this is the most common reason the farm does nothing.
• Once the zombie is in, replace the trapdoors with path blocks. Golems can spawn on trapdoors.
• Path or slab a 7 block radius around the farm. Some builders needed 9 before the golems stopped spawning outside.
• Keep wood away from the lava. Signs and planks catch fire, and slabs placed above the signs stop it without hurting the rate.
• Do not put the boat too close to the villagers. Too close and the golems stop spawning, and there is a risk of the zombie converting a villager. Push it back a few blocks and centre it so all three see it.
• If the farm worked and then stopped, break and replace the beds. Villagers lose their bed link and a villager that cannot sleep spawns nothing.