Francois, the MoTeC specific comms cable is only required for M8 and M48 ECUs all serial numbers and M4 ECUs with a serial number less than 3000.
I don't want to come across as being insulting but have you checked the basics, i.e. wiring to the ECU, does it power up? Check the 5V and 8V supply pins on the ECU connector to make sure it is actually on. Is the comms wiring to the ECU correct? As you are using a serial cable you only need 3 wires to pins 21, 22 & 23 on the ECU connector, there is a drawing for this which you can download from
http://www.motec.com.au/m4/m4downloads/.
You mentioned that it is on the right comm port, I take it you checked this in Device Manager and then selected the correct comm port in the M4 DOS software by pressing F5 when at the Program Integrator menu screen?