I have purchased a jdm 4 port with the 5 speed manual, but it did not come with an ecu, or any info from the seller about, well, basically anything.

I have been wholely unsuccessful in finding the correct ecu part number that I need for this engine; everywhere just says to match what the car engine came with, but I don't know that.

There seems to be a very large quantity of ecus available for cheap on ebay, but very few listthe engine it is for, and the few that so don't seem very trustworthy.

Here is the serial number on the engine just in case it's helpful.

I would like to avoid the nuclear option of buying an aftermarket ECU (>$1000) like the Haltech, though that would technically solve my problem.

 

Where are you based? This combination was never sold in North America, so that limits your options. Try breakers in the UK? No guarantees a UK ECU will be compatible with another region car though.
Is this to put into another RX8 or some other project? The engine doesn't care what transmission its attached to, so if you have a 4 port automatic, you can reuse the ECU in the car. Otherwise... Can you trade it for a 6 port? Far more common and a vastly better engine anyway.