You need to setup your P550 to share Com4
Click start then Settings.
Open the System tab.
Click on the GPS icon
Set the GPS program port to COM4 <------ this is the port you use to let MM TomTom, Cachemate etc share the GPSr.
Set the GPS hardware port to COM2 baud rate 4800 <----- This is the port that the GPSr uses.
On the access tab make sure that the Manage GPS automatically box is checked.
Then all you need is to set Memory Map, TomTom and Cachemate to use COM4.
Don't forget TomTom and Cachemate can not open this port and you will need to open Memory Map first.
(You will not have an option in TomTom for COM4 until you open Memory Map first.)