Well yes it'a a limitation when you use the OT midi tracks to send PG. Really I can't understand why manufacturers put such limitations. Maybe there is something technical that we don't know about it.
Anyway there is another way to send PG. Goto the OT midi sync menu and you'll find the PG send and receive options where you can specify the midi channels also. So each time you switch a oattern in OT a program change will be sent to A4 which probably is what you are looking for. For pattern based machines this is the way to control them. However when controlling synths the current 4 part-PG limitation is completely unjustifiable to me.
And again one thing comes in mind which makes me want to ask again for 1 PART PER PATTERN!!!