I use an MT4 with a Mac Book Pro, with Logic 8.0.2, logic 9 and 10.5.7, without any problems. I have a Mackie control System (MCU, XT and C4) running with my Mac Pro through a Unitor 8 Mk II and AMT 8 - I have tested this with the MT 4 and it seemed to be OK.
Did you install the USB MIDI driver from the Logic 8 install DVD? This one is required to operate the MT4. The driver is the same for the entire range of Emagic USB MIDI interfaces (Unitor8, AMT8 and MT4). You can install the driver over an already existing Logic installation by doing a custom install from the Logic DVD, and deselected all items except the driver (which is found in Hardware Support category of install items).