One other thing you'll need to swap is the fuel rail. The fuel rail has the gas line coming on the other side on the 5th generation so use your 4th generation fuel rail. The injectors on the 5th generation engines are saturated and on the 4th generation they're peak and hold injectors. The difference is that the saturated injectors have higher resistance and since the Honda ECU need that high resistance you need either saturated injectors or peak and hold injectors with a resistor pack. What the resistor pack does is put more resistance in the injector's wires to give them the same resistance as the saturated injectors. Cause the resistor pack is already wired on the 4th engine harness, it's easier to use peak and hold injectors. You can either modify the wiring or use peak and hold injectors. That's because you'll also have to swap the engine harness while the engines are on the floor.

Now swap your engine harness, first remove them from both engines, and if possible put some masking tape with something written on it that'll tell you where they go. Then put both harness on the floor or table or anything big enough so you can put them both side by side to compare them. Start with the main connectors, they're not alike but that doesn't matter. What matters is that those connectors will match the ones in the car, that's why you're using your old engine harness. After the main connectors, there's some starter wires, some sensor right on top of the transmission and we get to the distributor connectors.

You will notice that one of the connector is the same but not the other one. That's because on the 4th generation there's a sensor in the distributor that is not in the 5th generation distributor. So where is it? If you look on the back of your H22A4, there's some wires going down the timing belt cover and entering in the engine. That's the sensor. So here you have 2 choices, either use the distributor on the H23A1 and left the sensor on the timing belt cover unplugged or replace the connector on your H23A1 harness with the one on the H22A4 harness and lengthen the other wires to get to the sensor on the timing belt cover. Also if you want to alter your timing you need to use the distributor from your H23A1 because the sensor on the timing belt cover won't allow you to alter the timing. So the easiest way is to swap the distributors.

Also there's some coolant temperature sensor that's missing on the 5th generation the one connected to the upper radiator pipe connector on the engine. What we did here was swap the metal thing that connects the pipe to the engine from the H23 so you now have that sensor.

Now we get to the 2 sensors under the distributor, no problem there. After that there's a little difference. You will notice the coil wire isn't at the same place on both harness. That's because the coil is a little more in the back on the H22A4 to clear the VTEC solenoid and oil pressure switch, so you will need to lengthen the coil wires. Just cut the coil wires on the H22A4 harness about 5-6" away and replace the H23A1 coil connector with it. The 2 connectors are identical so it's not a problem.