While using a converted ATX PSU (or an ATX PSU with an adapter) only 1MB Fast RAM is detected per bank, so 16MB RAM shows as 4MB total, all other functionality is working on the board:
Cause: _FAIL (Pin1 – Grey) on the A4000 motherboard power connector is not wired up correctly to the ATX PWR_GOOD pin (Pin8 – Grey). If using Ian Stedman’s excellent ATX adapter design from 2016, the PWR_GOOD signal is mapped to the PCB side as a trough hole marked “fail”.
More info (curtesy of Tristan Zondag): The POWER_OK signal of the ATX have to be connected to the _FAIL pin of the 4000 PSU. RAMSEY uses this signal to initialize properly. If it is tied to +5v, RAMSEY will read the 256k/1m ram jumper incorrectly leading to the fault above.