Hello and welcome!
Using the Mk4 Haynes book on a Mk5 will probably only let you into how Seat etc "do things" - so while the information will not be too good for you, it might give you clues as to how things were done and in some cases still get done. We are now 5 years into the production phase of the
Mk5 Ibiza, so if Haynes are going to provide a book they might do soon. Haynes book for VW Polo 9N was first printed in (USA!) in 2007 - the Polo 9N first went on sale early 2002, so were there is interest, 5>6 years after launch seems to be the time. Though in that case, Fabia appeared a year or so earlier and a lot of the Polo 9N book was sourced from Haynes Fabia book. Also I'd expect the previous
Ibiza book was lifted from the Fabia > Polo book. Maybe contact Haynes at Sparkford and they will give you an indication if it is on the way.
When I bought a Polo 9N in late 2002, I bought the Fabia book that came out in 2006 to keep me going until the Polo one came along year or so later.