At this moment it is uncertain. There are images of one filing cabinet in Poliphony headquarters with the "Seat" brand on it and a non confirmed true image of the menu in GT4 Preview book (where it appears an icon "Spanish cars").
But all is very speculative at this moment for this question. There are not any image of Seat cars in the Poliphony test days and there are not any screenshot of the game with a Seat.
Ah! Seat is not spanish-german. It is a spanish company inside the Volkswagen group (german)
I hope SEAT is in as I own one & I quite like most of their cars.
Still I don't think my car will be in as its an older actual SEAT design, well if you don't include the Fiat Strada chassis, System-Porsche engine & various other VW & Fiat parts 70BHP is pretty good though for a '93 1.2!