Right. I apologise in advance as, in order to answer this, I am going to have to waffle on for a bit, but you did ask...
There was long standing confusion about the nomenclature used by Prodrive for the Subaru WRC program, specifically relating to the earlier cars.
Prodrive used an internal "S" designation for their all Subaru rally cars from the outset, but it was only from the very late 90's when those designations started getting widespread use in the public domain. At that point it was known that the 1999 season car (WRC99) was referred to by Prodrive as the S5 WRC. Subsequent cars were assigned sequential numbers...
WRC2000 - Impreza S6 WRC (also known as the P2000)
WRC2001 - Impreza S7 WRC
WRC2002 - Impreza S8 WRC
WRC2003 - Impreza S9 WRC
WRC2004 - Impreza S10 WRC
WRC2005 - Impreza S11 WRC
WRC2006 - Impreza S12 WRC
WRC2007 - Impreza S12b WRC
WRC2008 - Impreza S14 WRC
...the confusion came with the earlier WRCars (i.e. 1997 and 1998). In the absence of anything concrete from Prodrive - and applying the logic that each successive car has gone up by one - the presumption was the 1998 WRCar was S4 and the 1997 car S3. With Prodrive being very tight lipped about anything Subaru WRC this mistake was adopted by the internet and with sites like eWRC using it you had privateer car owners repeating the error.
When Codemasters licensed the 1998 Impreza WRC for Dirt Rally 2.0 I presume that they approached Subaru (sorry, SUBARU before PJ corrects me!
) with "S4", having been told that by the owner of the car they scanned for the game. Subaru of Japan wouldn't have known any better as the "S" was a Prodrive thing, so the car was licensed for DR2.0 as "Impreza S4".
It was around this time when more archive Prodrive material was becoming public and it became apparent that S4 wasn't right. At this point several ex-Prodrive guys were happy to speak about it and it turned out that they had mentioned the mistake in various corners of the internet over the years, but no one had listened.
They confirmed that the WRC97, WRC98 and WRC99 cars were all referred to internally as S5. Although the car had evolved across those three seasons, the internal nomenclature had remained unchanged. It was only with the WRC2000 (which was a complete ground up redesign) that a new "S" number was assigned.
It was also confirmed at that point, that Prodrive had applied an "S" designation to their earlier Subaru rally programs:
S1 - Group A Legacy
S2 - Group N Legacy
S3 - Group A Impreza
There is ton of evidence to support this available now, as the vast majority of Prodrive parts in the part manuals for the Impreza WRCars are in the format S
X-
xxx-
xxx; these clearly show Group A parts with a S1 or S3 prefix, while all WRCar parts are S5 or higher. In my own travels I have now seen Prodrive cage ID's on 1997, 1998 and 1999 cars: they all have an S5 prefix.
The correct information has (pleasingly) found its way around the interweb and to a lot of sites, including eWRC, who have updated their records accordingly.
That leaves one question: where is the S4? Turns out there wasn't one: Prodrive skipped 4 and none of the ex-Prodrive folks know why.