IE9 has implemented more of the standards that developers use and value, IE9’s Acid3 score has continued to rise, whereas Platform Preview 3 received a score of 88/100 on the Acid 3 test while PP4 scores 95/100.
Dean Hachamovitch, IE General Manager says:
With the fourth Platform Preview, Microsoft recommends developers, designers, and partners to start getting your sites ready for the IE9 Beta. Platform Preview 4 is an important milestone on the way to beta. It is the last preview before the IE9 Beta.