David,
Without looking carefully at the harmonic signature it's very difficult to come to a definitive conclusion. In simple terms, regardless of whether or not it's turbocharged, a 4-cylinder engine will be dominated by even engine orders (2,4,etc.) whereas a 6-cylinder will be be characterised by odd engine orders (3,6,etc.) - that's why, as you say, the 4-pot is perceived to have a lower engine note than the 6. Unfortunatley the friend I mentioned in my previous post has suffered the bereavement of a very close family member and therefore has much more important things to attend to at the moment, but I'll post any findings here when he's able to get back to me.
I'd agree that it would make perfect commercial sense that the Spyder and GT4 will share the same engine, and if the Spyder is to use the F-4 turbo engine then it would follow that the GT4 will also use this engine. However, the Spyder is not a GT car (i.e. not developed by A.P's group) so anything is possible, and although there's a lot of aimless chatter about using a "detuned" GT3 in the GT4 we've said on a number of occasions here that it's quite possible that a stroked (not bored as the PH posse seem to think) version of the old 3.8L engine could be used with little modification. Plus, although there are a lot of Spyders running around, we haven't yet seen a similar number of GT4s (conspicuous by its absence from the 'Ring perhaps?), so maybe there's still some way to go on GT4 development.
Jeff
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