Kent
With the PS3's power all of the problems above would be destroyed and the online mode would work without any sort of trouble from the console.
I'm not too sure about GT4 being a 'blueprint' for GT5, but it sort of makes sense. PS3 should be out within a year or so (well shorter than the life cycle of a GT game), and Polyphony was under pretty big pressure to ship GT4 (a year behind schedule, which means over-budget), obviously leaving a number of things unfinished (online) or poorly tested (changing units only changes them in a few places). PD figures, ship *something* now, and make some money while they continue working on the game. Leave out online play, as they know that it'll be a huge selling feature, and a reason for people to buy GT5 only a year after they paid for GT4, when the PS3 comes out. Plus, it saves them the headache of having to support online play for PS2/GT4 when the PS3 comes out (i.e. save a ton of money).
I highly doubt that online play was left out for technical reasons related to the console (the network, maybe). In any network game, each node can process stuff a bazillion times faster than it can communicate with the server and/or other nodes. Lots of FPS games have online play on the PS2, which requires just as much processing, and just as lag-free connections as GT4 would. But, for those FPS games, the 'single player' mode isn't generally interesting to keep someone's attention for very long, so online play is absolutely necessary. GT4, on the other hand, has the fact that even playing by one's self, the game is never really 'over' ... so while online play would be cool, the game can survive without it.
There you go, Kent, that's my $0.02