Can a ECM4 be used in place of a ECM3.

You have to either repin the harness, use an adapter to convert it from a 3 to a 4 harness or get a new one. The adapters are not much cheaper than a new harness. It depends on what kind of shape yours is in and how old yours is as to which is the best way to go. The knock sensor is also different for the 4.
Eddie

I assume you are talking about the Delphi MEFI3 and the MEFI4.

If you are using the 3 there is no compelling reason to make the change to the 4 unless it is fried and you can not find a replacement.

If I may be so bold, why do you want to change? The 3 can be tuned.

But if you must change re-pinning will accommodate the 4.

Same knock sensor works on both. You can import the mefi3 ptp file to the 4, although there's still some area's where you need to fill in the blanks.

Dustin

I was tolled that very little can be done with the ECM#3.

I am not sure who told you this but there are over 1800 point so data in the MEFI 3 that can be tuned. Typically you only need to deal with a very small set parameters ie... go to http://mefituning.com/mefitable

Picture lower right and you can see a sample of the fuel map.