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.